I had the same question and I would say that these sites are pretty good:
All those are quality sites that tend to have the lowest prices on mods.
As for boost gauges......hit up your local Kragen/Shucks/Checker Auto Parts store. At one Checker store I used to live near, they had a bunch of Autometer gauges on sale. That and if one goes belly up, which happened to my wife's oil pressure gauge, you can get a replacement one quick. As for pods for the gauges.....you could be SOL depending. Just order them from the above mentioned sites.