Either pay for them as you find them graded or gamble with a bunch of played ones grading them. 1 is easier than the rest I think as any blastoise with a couple bad folds would get a 1 no doubt. 2,3,4 are just guesswork. Check out the 1-10 charizard collection @xgav1337x has and maybe you can get some clues. I think he had it a bit easier as charizard are graded much more often and I think he bought several of them.
www.elitefourum.com/t/base-set-charizards-psa-1-through-10/14677/1