If you went to an "aquarium store" (pet store) you wouldn't be looking at a pond pump...but a fish tank pump
Watercooling started with using aquarium pumps...so you might find something there. But you probably won't find a 12v pump...so it'd have a 120v/240v AC connection. Also, like meionm said, these pumps aren't as efficient as pumps specifically used for watercooling.
I just went to PetsMart.com (not sure if that's a nationwide place or not, but they have a local store by me) and found 2 pumps that are used by ppl for watercooling. Keep in mind that these are 120v pumps so you plug then into a wall outlet, not your power supply.
EHeim Pumps
These pumps are pretty popular for use
[url="http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441808186&FOLDER%3C%3Efolder_id=2534374302030157&ASSORTMENT%3C%3East_id=2534374302023693&bmUID=1149461001615&itemNo=4&Ntt=hydor+pump&In=All&previousText=hydor+pump&N=2]Hydor L Pumps[/URL]
The L30 is fairly popular for watercooling
If you are looking offline because of issues with not having a credit card or something like that you could always goto a local bank or other place that sells Visa gift cards. You use them just like a credit card. I think they will charge you like $5-$8 ...but you'll probably save that much by buying online. And then you have the option of getting a DC pump (if that matters to you)
Although, with a little snooping it looks like you may be located in germany? If so, I couldn't tell you a specific place. But I'd think those pumps would be avaiable at a pet store there too. ?