Look in the OSR Deals thread, I know I've posted lower parts a few times since Novemeber, mostly from KM Tactical. They're hit or miss on other parts, they sell a lot on GunBroker too if you'd rather buy through them for some reason.
AIM has had their house brand parts in stock fairly frequently. They only do RMR cut as far as I know, but I've already got a Burris Fasfire so I kept looking.
I got a Swenson Fastfire/Venom cut 17 slide and barrel from Midway on sale for cheap a month or so ago. I don't know anything about Swenson, but it seems good enough for me.