Cougar:
The VP-30 pump is available from most hobby suppliers, here's the home page for Perry, part of Conley Precision:
http://www.perrypumps.com/
You can also find the carbs listed there.
Using the big carb is good or bad, depending on the application.
If you need max rpm, the pump carb will raise the available speed a good percentage over the stock carb, it's like putting twin carbs on you hot rod, lets it breathe more freely. And it needs the pump.
But if it's a large plane that needs a big prop at lower rpm, you'll be burning more fuel without going any faster. The smaller bore carb just works better at the lower rpm. And does not need the pump.
In either case, though, if you want to put the tank on the cg, or any distance from the engine, the pump does very well.
My C-3/10 twin uses a single tank in the main fuselage, each engine has its own pump. See picture.
Bill.