I've been told I need to change out the extractor in my M1A SOCOM for a USGI (as well as the spring).  Where would be a good place to get these parts?

You do NOT have to change out the extractor for a USGI extractor. This is a common M14 A-hole "expert" piece of advice.
You should have another extractor and spring w/detent on hand for when you need it. This goes for USGI parts or aftermarket parts. The extractor is the most likely part to break in the M1/M14 design.
Again, do NOT change it if it still is working; but, have a replacement on hand.
Parts such as this show up on the Equipment Exchange from time to time. Also, odcmp.com has them for sale in their ESales section.

They pop up on the EE all the time. There are other websites that sell them too, but you'll generally get a better deal off the EE or even the M14forum PX.

I guess I'm also one of those "A-hole" M14 guys that likes GI parts.

me too!   There was a time when there were a lot of guys on the web wondering why their rifle ejected their bolt parts.  I think the new ones from SAInc. don't have this issue any more.  
No offense to AO guy.   I do believe this was a period of time some years ago with cast aftermarket extractors and even long shanked usgi ones.

That said, if you don't have any issues, then by all means leave things alone.  No sense in fixing what ain't broke.  A modest spares box is always a good idea though.

Extractors them selves in SAI's M1A's are not prone to breaking, just in M1A's that were built in the last 20 years. Older SAI M1A'S were built with USGI parts,not so anymore. The extractors are commercial made and are prone to breaking, its a well known issue. USGI M14 extractors are not prone to breaking and will last a lifetime. IMO I would change it
http://www.treelinem14.com/Bolts-and-Bolt-Parts_c10.htm

I have had great dealings with Claude at RAparts.com

I don't know how the prices compare, but http://www.treelinem14.com/main.sc has a good selection of USGI parts.

I've used them numerous times and have never been disappointed.

Even USGI extractors do not last forever (i've had three break over the years). Consider them like tires on your car. USGI will last a long time before needing replacement (like high cost tires). Commercial will probably not last as long (cheap tires).  Who is going to replace cheap tires that they already own for really good new tires? Drive them until they need replacement and have new ones of your favorite kind available for backup.
As of this minute, there are two USGI extractors and detented springs on EE for $20 each shipped. There are also (on www.odcmp.com) USGI refinished extractors for $12 and springs for $5 plus shipping. The part numbers are: PS15APW and PS1617APW. For about $20, buy either and swap it into your rifle or wrap it up and put it into the rifle butt storage cavity until needed. Either way, use one and keep the other until needed. Another handy item to have on hand is an empty .30-06 case. This holds the bolt about a half inch from locking in the receiver while you replace the extractor while holding the ejector in place.

A buddy from work had the spring break in his and the extracor fell out. He lost the plunger. I ordered one from treeline.com(?) It was a USGI replacement and was pretty cheap.