You only need to find one pumpkin, as you can now grow them. (which might not've been true when you first asked this). So, provided you haven't already turned them all into jack-o-lanters:
- place a pumpkin into your inventory, and get seeds.
- place two blocks of dirt
- till one of the dirt blocks
- plant the seed
After a while, a plant will appear ... leave it alone. It'll get darker, then some time after that, a pumpkin will appear on the patch of dirt next to the vine. Break the pumpkin, but leave the vine/plant part, and a few one will reappear in less time than waiting for a whole new plant to re-grow.
And, as with all plants, it'll grow faster when it's within 4 blocks of water ... so I generally set up my little gardens with a square of water in the middle, then 4 blocks to each side of it as tilled dirt (so a 9x9 square), with pumpkins and watermellons planted on the outside ring (with another ring of untilled dirt for the melons/squash to grow on, as they won't form where there's something else planted)