I know a man who works for the council and lucky for me they are slowly replacing all the pavements. But I have to take pot luck as to what they have but as I only have about 20 at a time I can usually get what I want.
The slabs you get from garden centres aren't up to the job, these ones are very heavy and really bed down and don't move.
Try contacting your local council depot, don't bother going to the council offices you need to get to the depot, usually where they store all the road signs, grit, council vans and lorries etc.