Thought Olympic had closed their doors, but apparently they're looking for a buyer now. Looks like you can now get 22-250 bolts and extensions!
As far as 9mm ar15's I'd go with a dedicated 9mm lower (I like the CMMG colt pattern) over the magwell blocks which seem to have more issues with feeding. Keep an eye out on midway as over the summer they had AR-Stoner 9mm barrels on sale for around $60 like this one https://www.midwayusa.com/product/10...moly-phosphate PSA or Spinta will occasionally have blem ramped bolts for $100 or less, get a heavy 8oz-10oz 9mm buffer and the rest of the gun can be standard 223. Get a 9mm port door and deflector if you wish, but a 223 one will work. Jse usually has 9mm port door kits for less than $15 http://www.jsesurplus.com/cmmg9mmdeflector.aspx and the c-products 9mm mags work great for me http://www.jsesurplus.com/cproducts9...agazine-2.aspx
We got one of the AR-Stoner 9mm barrels for $60 back when they had their 4th of July sale as a gift for the FIL. Spinta had a run of blem 9mm bolts at the time for $90 and I made him a heavy SS buffer using a piece of 304 SS and a $1 KAK industries buffer bumper. He didn't want a dedicated lower so we got him a KAK magwell block https://www.kakindustry.com/ar-15-pa...mm-mag-adaptor handguard and muzzle break off of ebay along with a standard upper, the JSE port door kit and a standard upper. The magwell block fit too tight and needed .005" taken off of the front side and to make it feed easier the feed ramp in the barrel was recut, but a slight polish of the mag block bullet guides and feed ramp would of likely worked also. Works great. Usually I make my own 9mm barrels, but for less than a $20 difference from a GMB gunsmith blank the ar-stoner barrels are a steal.