I only know of one Panda with 15's (sorry I can't remember thier username), they are Fiat barcetta steelies with 195/45s I believe. He had to roll the arches to get these to fit.
I run 14's with 185/55s that don't rub, however wheel offset is critical to get the right clearance and different tyre makes can even make a difference (side wall bulge).
In thoery any front wheel drive Fiat/Lancia/Alfa 4 bolt wheel should be ok but no wider than 6 inch max (5 - 5 1/2 is best) and a seletion of spacers to get optimium offset/clearance.
I think you will stuggle with the large profile of an offroad 15inch tyre on the Panda ( a 185/60/14 is about the max hieght on a 14 for comparision )
best of luck Damon