Bought this one on E-bay. Cheaper than Dirt has them too (with reviews).
This one is for a flat-top. They illuminate red or green with variable brightness or can be used "off" for during the day (plain black recticle).
This one is for a carry-handle. You can read reviews here (they're both the same scope):
http://www.cheaperthandirt.com/SCP314-46173-1512.html
They'll leave you money for getting accessories and come with warranties. I wouldn't use it in combat, but it's good enough for messing around.
Yeah, it's made in China and it's not "top" quality, what can I say. I'll admit it, I shop at Walmart.