If you are in the USA I can't help as to where to get the varmint version of the tikka, but in my area (Toronto Canada) pretty much all the local stores, cabelas, bass pro, all have or can order a varmint barrel. I'd be surprised if there was less selection at USA retailers.
I am in Pennsylvania to be exact. I picked up the T3 300 WM in stainless steel yesterday at cabelas. Every shop that i have called say the heavy barrels are not available or say they can try but don't know how long it will take. No big deal, will buy a savage 10 or a remington 700 long range for their heavy barrel in the next couple of days as well. Thanks for chiming in by the way.