Ambi Bolt-Release Lower Receiver....

[Adog5]

The Robinson XCR is a an ambi bolt release, but I get the feeling you're looking to build one and use AR-15 parts. But just FYI.

[Blacktail 8541]

There is anothe alternitive altho I don't like it as well. Scroll about half way down.

http://www.compasslake.com/parts.htm

[SunriseF150]

Unfortunately those Magpul lower will probalby never come out. They posted on AR15.com recently that those are on the backburner. I wish they would just give the design to a company to have them produced for them. They have too much going on as is to try to machine them themselves that it will never happen.

[supersonic]

Bushmaster makes a release (they actually are the original inventors/manufacturers) that I put on my competition / long range AR, and I don't know how I ever lived without one before! You don't have to move at all or make some awkward adjustment that will put the rifle off target. You only use your trigger finger. AND, it bolts right on to your existing BC w/ a set screw. Scroll down 3-5 items:http://www.bushmaster.com/products.asp?cat=16
It's not cheap, but you do get what you pay for. Hope that helps!
Scott

Edit: the Model 1 is a copy of the Bushmaster. Not even close in quality. Know 2 fellow shooters who, after breaking their Model 1's, went with the Bush. Luckily, that's how I came to find mine. Now, I won't have to buy 2.

[ar15barrels]

Mine came from RB precision about 5 or 6 years ago for about $20.
I'm pretty sure they make them for Bushmaster.