The only MP figure available at retail right now is MP Rodimus which is a TRU exclusive. As for the others, you'll have to resort to any local toy shops/comic book shops that might sell things like that... oh wait you said cheap, scratch that last idea then When that dries out there's always Ebay, Junkion Exchange, BBTS, TFSource, etc.
Masterpiece & cheap don't really go together. Yeah, try local hobby shops, you might be able to score something at a "decent" price. I recall a couple of months ago, my local toy/hobby shop had an MP-01 Prime for $100 MISB. Till this day, I hit myself for not buying it as they're never below the $200ish asking price..
It was really spotty about 6 months to a year ago. One or two popping up at random stores around the nation. Best bet is the B/S/T thread. Some of us have Skywarp in our sales threads in the links in our signatures
I have a Masterpiece Optimus in my own B/S/T thread. However, I am currently in discussion with someone about it. I'll keep ya in mind if those talks don't pan out.
Outside of Rodimus and a few random bargains this is as cheap of a master piece as you will likely find : Amazon.com: Skywarp Transformers Universe Masterpiece Exclusive Action Figure: Toys & Games