When I start players at high levels I let them buy whatever they want of non-magical items. Particularly at level 15+, they should be fabulously wealthy at this point (fighters having build keeps, thieves establishing guilds, wizards building towers, etc).

For magical items, if you're looking for a by the book way of equipping them, I would use the tables in the Random Encounters section of the DMG (p. 194, "Magic Possessed by Encountered Creatures")

These tables give a decent and class-appropriate distribution of magic items.

Or you can just pick them.