There are no heatercores in 15 by 15 sizes, that I know of. I can't find 5 by 15, nor 7.5 by 15, but 7.5 by 7.5 is possible. 5 by 5 is also available.
I'm first going to point you towards an unusual heatercore:
2-636
It's 9 1/4 by 3 1/4 by 3 3/8 inch. It's from a VW Fox. It's small, compact, but very thick.
There's also 2-531, 20 1/2 by 3 1/8 by 2 inch. Ford.
Koslov: are you sure that that's not a mistake? The graphs are identical.