Sounds like your best option is to put a 120mm in that rear hole.
You can buy 90mm fans, they make them and there are plenty on newegg.
But newegg doesnt ship to croatia
If you can't find someone to ship to Croatia, you can send me the money and I'll buy them and ship them overseas... most likely it's gonna be real pricey... never shipped anything overseas lol.
But in any case *pun intended* get yourself a 120mm for that back.
I would suggest a pretty high CFM fan if you are only going to add that fan.
If you buy some 90mm for the sides, you won't need a real high CFM exhaust.
That case is designed to run all exhaust, so the air filter in the front of the case draws in the air and gets caught on the filter.