Is there a site that will allow you to create custom plastic snow globes online?

SnowGlobeCentral makes one of a kind custom glass snow globes.

If you are looking for inexpensive plastic snow domes or globes, you may wish to buy the type of snow dome that allows you to size your own photo, and slide inside a slot in the base.

If you do a search for "photo snow dome kit" you'll find many options. but Amazon, Sears, and others also provide empty snowdomes to be personalized with your photo.