Does anybody know where I can pick up 20 rd.Thermolds for my AR?

Try the "[b]Equipment Exchange[/b]" up at the top, go to "[b]mags[/b]" and shop till ya drop.

Mike

PS - Welcome newbie...

The Master Mold 20 round thermolds are rare.  While the 30 rounders will pop up all the time, the 20 rounders pop up once in a while.  Check with Computer Guy.  I think he has the ever more rare Orange Thermold.

This is from www.ecis.com/~mraudio/FAQs/Mag_FAQ2.htm

[3-2] Plastic/Nylon/Polymer Magazines
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Quiet
No finish
Bulky

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[3-2-1] Orlite
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Orlite magazines were developed for the Israeli Armed Forces. The bodies are made from high-melt-temp black nylon with a steel reinforement mesh imbedded in the top 1.5 inches to strengthen the feed lips. New Orlites come with a rubber cap to help keep sand out of the mag.
Orlites have a rib around the middle of the mag which helps to seal the bottom of the mag well and also prevents the magazine from being over-inserted and the feed lips damaged. The location of the rib was based on the deep mag well beveling on Colt M16s, and in ARs that aren't beveled as deeply, early Orlites won't seat completely. This is easily correctable by trimming a bit of material from the top surface of the over-insertion rib; just enough to allow the mag to seat. This problem only occurs with early Orlites with mold numbers from 0/1 to 0/19. Starting with batch 0/20, the rib was moved about 1/32" further down the mag body, and these later mags work in all ARs with no modification. Used Orlites are often found at gun shows with their over-insertion ribs completely ground off. Avoid these mags, as it's too easy for these mags to be over-inserted, banging the feedlips against the bolt or getting stuck.
Orlites are quite reliable, and were general issue in the Israeli Army through the 80s, but it was found that they don't hold up to hard combat use as well as USGI mags, which Israel switched to in the 90s.

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[3-2-2] Thermold
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Thermold magazines were developed by Master Molder in Wilson, South Carolina, and were "pitched" to the US military as a replacement to aluminum "USGI" mags, but the US military was unable to come to an agreement with the owner. Master Molder then licensed the design to the Canadian military, who manufactured a version of the Thermold 30-rounder as the standard-issue magazine for their military.
Thermold magazines are made from a dark gray polymer and have anti-tilt followers. They have several reinforcement ribs across the lower half of the mag body.

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[3-2-2-1] USA Thermold 30s
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The most common of the Thermold mags, these were made using DuPont Zytel, a fiberglass-impregnated plastic that is very strong and has a high melt temperature. Mold markings:v
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[3-2-2-2] Canadian Thermold 30s
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The Canadian military licensed the Thermold design, but did not use Zytel as the polymer, using a less expensive plastic as a substitute.
This was not a wise decision, and the Canadian military had problems as a result. The most famous problem was the feed lips melting when their ARs were fired for extended periods on full auto (especially with blanks), leading the Canadians to dub the magazines "Thermelts". The other common problem was the mags splitting or cracking in cold temperatures or when dropped. After Desert Storm, the Canadian military changed to USGI aluminum mags, which are now standard issue.

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[3-2-2-2-1] Canadian Thermold "Blank" 30s
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The Canadian military made some Thermolds with bright orange plastic as "blank only" mags. These are relatively rare, and are made with the same plastic as the standard Canadian mags.

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[3-2-2-3] Thermold/Master Molder 20s
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Master Molder made straight-bodied 20 round mags. These are US-made from Zytel and are the best aftermarket 20s. Mold markings:

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[3-2-2-4] Thermold 30/45
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Among the most unique magazine designs, the 30/45 magazines were designed with an extendable bottom section that would allow the magazine to be fully loaded to 30 rounds and stored (with the base extended) with the spring under minimal pressure. This allowed long-term storage of fully-loaded mags with no wear to the spring or the feed lips. They can also be loaded with 45 rounds when the base extended, though they shouldn't be stored in this configuration.

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[3-2-3] Ramline
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Ramline (now owned by Blount) manufactured a 30-round magazine that was designed to work in both AR15s and in Ruger Mini-14s. The mag body and follower are made from a transparent yellow/brown plastic and the spring is the standard coil-type. Because the mag is longer front-to-back compared to a GI magazine, the Ramline mags are an extremely tight fit in AR15 magwells, and often require some sanding to fit. They tend to fit better in Mini-14s. The plastic used in the mag body is brittle, and the feedlips tend to crack and break over time.

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[3-2-4] Eagle ------- Eagle made a 30-round magazine which they called "Beta" (not to be confused with the Beta-C drum magazine; the companies aren't related).
Eagle magazines are a dark grey transparent plastic, and they have ribs on the bottom similar to Thermold mags. These mags use flat-wound "constant-tension" springs attached to the front and back of the follower, similar to the design found in most high-capacity 10/22 mags. The plastic mag bodies are brittle, and crack easily, especially at the feed lips.

I have 3 Thermold 20 rounders I will part with for $90 plus shipping for all 3 mags.They are in new condition.I have never used them.If you want them,you have to purchase all 3.Let me know if you are interested.