This is probably the smallest pump you'll be able to find - its an inline reciprocating pump powered by 16 VAC, 2.75" long x 1" diameter, and pretty powerful for its size:
I'm not sure if you can get these anywhere else besides Edmund's; their stuff always tends to "feel" overpriced - and this pump is no exception.
Something you have to keep in mind when it comes to water pumps is "small size" and "high pressure" tend to be inversely proportional - you should see the size of the pumps needed for the Fountain Hills, Arizona fountain:
...at one time, it was the highest shooting fountain in the world (now fourth highest). When that sucker's running, the ground vibrates.
