Where can I find a replacement part for a seatpost collar?
Through a lot of searching, I can't say there's anywhere that sells that specific part; however, you may have some luck contacting the manufacturer or distributor directly.

But, a much easier and simpler solution would be to replace the clamp/collar entirely. It's probably pretty worn out if you're missing that piece anyway!

Check out the great selection over at Jenson USA, or your favorite LBS.