DS Arms is proving to be a reliable backorder source. Their prices have gone up a bit, but not nearly as crazy as other sites. They "actually" answer their phones. I just called them to add items to my backorder. www.dsarms.com
I called and confirmed today. They are 4-months backordered on uppers, bolt carrier groups, and firearms. If you are looking for parts and are not saved up for an immediate purchase, consider backordering with these folks. They won't charge your card until the order is pulled and shipped.
If you have to wait for parts to "build" an AR, you may consider just buying one of their completed rifles. $788 + shipping is not a bad price.
Palmetto State Armory has been amazing. They are FAST on their email notifications. Shipping takes a 2 weeks, but not bad. The best part is they seem to be getting a LOT of stuff in stock and on a regular basis. If you are building a custom rifle, get on their email list.
http://palmettostatearmory.com/
MidwayUSA is also pretty good on their email notifications. They ship FAST, too. The only thing that bugs me is they are slow in getting in product. When it is in, they sell out FAST. You are competing against a larger custom base with MidwayUSA.
http://www.midwayusa.com/