eBay, my good man. It's been a couple of years, perhaps they've tightened up a bit, but you may still be able to find them. I believe I paid around $30 for a pair. There were a TON of them when I got mine, I only see one on eBay right now and it's already more than I think I'd pay.
If it's not numbered to 10 and doesn't have the prancing pony on the bottom, it's not Colt brand and you ought to avoid it.
If you have a magwell, you may have to trim the black plastic base cover a little in order for it to seat fully.