I understand that your lights are 1250, yeah? With that power and having the gel attached at the end of the barn doors, your gels will last a long time. By a long time I mean months of re-use (true for rosco, lee filters have a thinner base and show discoloration and warping much faster than rosco).
As for the original question about the sizes in which gels are available, you should check with your local film commission to find out about suppliers of expendables. Companies that cater the film industry carry the gels on rolls. You can "rent" the roll and, when you return it, you get credit for the unused portion.
I hope this helps you.
Cheers,
Rafael Baez
Mole Richardson doesn't make any 1250W lights but there are some smaller manufacturers that doSmith Victor and the IFF Qlites are 1250.