Are there places where pumpkins are likely to occur?

I've found a number of pumpkin groups in forests, others (like yourself) have found them in grassland and snow. Unfortunately, the code for generating pumpkins looks like this:

if (rand.nextInt(32) == 0) {
    ...
    (new WorldGenPumpkin()).generate(worldObj, rand, i14, i16, l18)
}

Which indicates a 1/32 chance of when generating a chunk . From there the code generates a 64 random positions within that chunk, and the positions that match a certain criteria (a block of air with grass below) makes a pumpkin. No mention of any particular biome at all.

Pumpkin generation per biome is random*. Sorry!

*as of 1.5_01

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:

1. place a pumpkin into your inventory, and get seeds.
2. place two blocks of dirt
3. till one of the dirt blocks
4. plant the seed
5. wait.

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)