What type of sand to use for paving mortar mix?

just use the plastering sand,i did and have no problems,done 5 years now,i mixed 4 to 1

Screeding sand is a coarser aggregate to plasterting sand. You can use plastering sand for a thin screeded floor, but for depth you need the proper sand for less cracks and strength. Its too coarse for rendering though.

Quarries and some good builders yards sell it loose.

Just be sure it's not paving grit which is very coarse and doesn't mix well with water and cement for laying slabs on. Plastering sand is Plastering sand all suppliers have it, just don't get the wrong stuff.