Slidetite can be purchased in a number of shops across the UK. There are also a few on-line and mail order companies selling it. If you cannot find it in a shop near you try some of these -->
Bak-Rak Banchory 4x4 Foundry 4x4 MASH Bearmach
Or do a Google search - there are quite a few links -->
Google search
You can also find a fair few people selling Slidetite on eBay -->