I used the dimensions 13.5"x 11.5" and found a panel that's used for a gate opener.
Solar Panel 10 Watt
I guess it could be used for a small attic fan too..
Edit:
I think maybe I have found the source. The back of the panel is marked UL-Solar.
I found the company and what looks like a newer version of the panel I've ordered..
10 Watt 12 Volt Multicrystalline PV Solar Panel - SPM010P-R