Most of us when looking for a replacement strap we go by searching a brand name then by straps, well then you won't probably get what your looking for as most strap makers will usually make straps more to a general fitting with custom designs.
Example, if i'm looking for a strap to fit my Hamilton or IWC watches i would look for any strap i like as a replacement that has a lug width of 22mm & buckle width of 18mm or 20mm depending on what custom buckle i would like to use.
below is something that could help you learn how to measure your strap.
as you can see the term "Lugs" is where your strap will be attached to & is usually measured end to end on the inside. So if you measure your lugs at 20mm then that will be the size of the strap for your watch.
Now some products will list as 20mm/18mm meaning 20mm lugs /18mm buckle width or 24mm , 26mm, 28mm, / 22mm , meaning 24 to 28mm will be the lug width & for all 3 sizes the buckle width is 22mm.
Now you have a Panerai luminor which you want to replace with a Nice alligator strap so here is a picture on how you should measure your watch lugs & buckle width.
Once you have your measurements, then you can start strap hunting. hope this helps!
regards
Strapmeister.