Not sure if there are better places, but I have ordered from Rockler before and used fans in a hobby paint booth and in cabinet for PCs.
http://www.rockler.com/product.cfm?page=6572&filter=fan
The nice thing is they are already wired for 110v..
They work well. I am also doing a custom cabinet for my HT build out... so this is a timely thread. I have done aerospace grade avionics cabinets, but scaling for something I can afford is a different story all together.
I may not know much about audio, but Wayne is right on target with his comments regarding cooling. Personally I prefer venting/installing fans at the top. This the way the heat wants to go and you want to provide the path of least resistance for it. If it is sealed from shelves above, I suppose that is essentially the same thing as having it installed at the top. You just don't want to create a heat trap.
The only issue with filtering is that these smaller fans typical of this cannot handle significant pressure drops across the filter and getting a decent filter that will get the fine dust out tends to work against you in terms of CFM and pressure loss.
But you have to do it. For my paint booth, I acquired some non-woven fabric at the local fabric store and have been using that. I also used "used" nylons and stretched/stapled them across a removable frame. I am looking for a better solution for this build out.
I am definitely interested in this thread... let me know if I can help.
Mitch