If you're looking for sources for 1911 grips in general, I would think a post in the 1911 Club would get you the most responses. Something like a "Where have you bought 1911 grips", or "Who is your favorite on-line 1911 grip dealer" should be okay.
Now, if you're wanting to solicit private sellers, then Eric wants all buy/sell/trade posts to stay in the Want Ad forums, so you'd have to pick one. Maybe the Gun Parts & Accessories forum? It gets a good amount of traffic. You could try if there first, and if you didn't get much of a response, give it a week or two and then try another forum. You don't want identical threads running in different forums at the same time, though; violation of the GT rules. I'd have to sic Scott or Eddie or somebody on ya.
Hope this was helpful.