I doubt their origin would add much value to the slate, they are after all just slate roof tiles and these can be bought new as well as used.
I just can't visualise anyone placing an extra value on these plain flat pieces of slate just because they came from the such and such church, it would be way different if they were stone carvings from the wall, bronze, stained glass, tower bells, copper finials or carved wood, but they are just flat pieces of ordinary stone slate roofing.
Try here, maybe one of the slate roof restoration firms would be interested inbuying them, but this stuff is very heavy, easy to break and costs a fortune to ship so who knows;
http://www.jenkinsslate.com/usedslate.htm