All I can say is keep checking Ebay for the Aeromaster sheet. Those old sheets do come up now and then. I've found them for Yaks and La-7s before. Another alternative would be this sheet from AML, which depicts another La-5 with a shark's mouth:
umm-usa.com/.../product_info.php
I see UMM is sold out of this sheet, but it might be easier to find than the old Aeromaster one. Try any UK online hobby shops, such as Hannants or Model hobbies.
Edit: Any shark mouth decals in the Eduard La-5 rebox??