Thanks Operator 5, the only ones I've come across so far (haven't looked too hard yet) are the ones which you have to dig out of a small plaster type block. I got 2 for my nephew when we went to the aquarium. Regretably I didnt pick some up for myself. It wasn't until I had to glue them together I had the idea and did a size comparisson. One we had was a good size ( a marine one(lost the name at the moment)) which even had a stand. The other was small but you could still use it for a smaller species.
If anyone wants the names and the manufacturers let me know (PMs welcome) and I will get them for you.
I'm just waiting to get them for myself.
Fritzy
PS I'm sure I saw them in Toys R Us as well