Farm foods, pound shops and our local Premier. Our local tesco garage only have them in rearly and off a strip in between shelves dangling. Morrison's do the same.
Sometimes not in an aisle and obvious.
Lots of firms make these local to where I live (west mids), the really light, puffy ones are called pork crunch, there are slightly more substantial ones called leaf scratchings and the really hardcore ones are just pork scratchings. All are low carb and personally I think the best pork scratchings are made by Simmons in Walsall.