I jusr recently got hired on with the local County Sheriff's Office. I carry my own personnel Springfield Mil-Spec 1911 .45 that is modified quite a bit. I was then issued a Remington 12g 870 with extened magazine and a brand new Ruger Ranch Rifle Mini-14 .223. It came with a 5rd mag and a steel 20rd mag. I am looking for a couple of 30rd mags for it and wondering if anyone knew of some reliable Mini-14 mags? I have seen some Thermold Promag 30rd mags for around $17 a peice and was wondering if these were some pretty good mags and would function well? Please let me know what you think. Thanks.

The only mags that I would trust for duty use with the Mini 14 are the ruger factory mags.   All the other are for range use, and to learn clearance drills.    
That's what I think.   Based on many,  mini 14's and many, mini  mags.   Say that 3 times fast..

Ruger factory magazines are the only way to go. Call Ruger and ask who the local LE dealer / distributor is. They can get you some. Ruger won't sell high cap magazines to anyone who's not an LEO. You may need a letterhead to buy them, but its no big deal.

I had a Mini-14 GB that was my first patrol rifle. I had to buy it on a letterhead, but I got the LEO discount too so it was worth the hassel. Ruger will drop the price on most of their guns for LEO purchase, but you have to do it through a Ruger LE dealer last time I checked.

Ruger only produces the 20 rd. mag anymore.  The factory 30 rd. mags are gonna cost between $75 and $100 each.  Several people have told me that the next best thing is the PMI mags.

Here is a place that has factory 20 rounders Link if you can find an LE discount you might be able to get them cheaper though.  

Ditto what everyone else said. Factory Ruger. The 20 rounders are available. You do not need 30 round mags.
If you need more than two 20 round mags in an engagement it is time to unass the area.
I use 20 round mags in my AR15.
Low profile is good when you go prone and shoot under cars.
20 rounders will serve you well and are available.
If you really can't find any let me know. I have a source.