well players do have their own spawners... like i personally have 2 zombie xp grinders, soon to be a skeleton farm, 4 spider farms, just go looking for a spawner and make your own xp farm o.o
a xp farm at spawn wouldnt work due to the sheer number of people who would be using it at all times.
I'm actually building a mob / xp grinder in the normal world. Problem atm is that it would need several mob spawners and a world anchor to work faster.
It is still work in progress and I would need 12 mob spawners or more to get it to go fast as it can, but I can't buy spawners.
Fastest way to get xp is finding a blaze spaner in the nether and claiming it so some griefer won't destroy it, but that may change if I can get enough spawners in the buildworld.
It's not the fall that kills you. It's the sudden stop.