Hey Matthias,
Check out bar sinks. They are small and may be close to the dimensions
you need. I did a quick search and found this link. Didn't find
anything fiberglass but I didn't searh to deep. They may be a bit
pricey but if you have a restaurant supply store close, they may have
soem used one's. I have one in my neighborhood. I'll go by with your
dimensions and see if he's got something reasonable.
here's a link to one thing that may give some kind of lead.
Good luck.
c