I shop
http://www.modelexpo-online.com, http://www.ehobbies.com, and
http://www.internethobbies.com and figure out which store offers the best price (after including tax and shipping).
Tamiya rules in 1/48 WW2 kits, but they can be pricey. Fujimi and Hasegawa also make nice kits, and Revell AG of Germany is good (which is now a separate company from Revell/Monogram).
I get most of my model stuff second-hand from eBay. I focus on 1/72 scale which tends to be pretty cheap anyway. If you're just starting out, I would suggest you do a second-hand kit and save yourself some teeth-gnashing over ruining a brand-new top-dollar kit. The more you build, the better you'll get, so start cheap and work your way up into the high-end stuff. Of course, its easier to build a good kit well than to try and turn a bad kit into a good plane.