First off think about what you want to do with them. Because they can get very expensive indeed. Good luck finding a Garand with anyone asking less then $1000 for it.
Though keep in mind, World War 2 guns were built to last. Even if their condition isn't mint. They can still be excellent shooters and might end up costing you less.
As mentioned before, The Mosin-Nagant 91-30 rifle is a great rifle to start with. It's cheap and it's a great shooter, abit heavy though.
Russian guns are easy to find. English/Canadian guns are pretty easy as well, American guns are fairly easy. German, so so. The toughest to find are Japanese guns. Tough to find and cost a fortune.