RE: Offical nitro tc3 thread
Well [link=http://www.lefthander-rc.com/catalog/index.php?cPath=24]Lefthander R/C[/link] usually has some.
The [link=http://www.bsr-racing.com/cap/]BSR[/link] site has a list of distributers. The link is to their Capped tire page.
[link=http://www.jagracingteam.com/index.htm]JAG Racing[/link] sells them at local events in Texas. There is a store link (ebay) on his site. Though I haven't seen him put BSR's online so he may not sell them remotely. But i've never seen cheaper prices for BSR than his.
If BSR doesn't end up being your thing then I would check out Yokomo, Take-Offs or HPI.