You can hollow them out and use them as a vase or hollow out the centers and add a votive candle for the center of your tables. Decoate them with bling -- Options are only limited by your imagination.
Photo from April Hiler Designs (faux flowers)
Where can you get white pumpkins? Last weekend Whole Foods in Plymouth Meeting, Pennsylvania had a huge stack of them outside, but I'm sure there are other places to pick some up. And if they had them in that store, they're probably widely available at other stores.
If you want to use a white foam pumpkin try the design by April Hiler Designs shown above. I'm not usually a huge fan of silk flowers or faux anything, but love this design! April has other great designs available in her Etsy Shop.
Photo from Wedding Party Centerpieces