There's always the option of buying "knackered" wheels very very cheap as a base to start. More often than not, it's the rims that are worn or damaged so you can re-use the hubs and all / most spokes. Then keep you eyes out for some right dimension
rims / hole count. Sounds like a faff, but you'll get some practice de-lacing, a feel for the materials, and keep the costs down
in your first attempt. When you re-lace a hub, make sure you position the spokes exactly like the original build. I pretty much
learned wheel building by doing several hub transplants before I built a fresh set of wheels; acquiring the skill is very satisfying as it's arguably the most advanced wrenching skill needed for a bike.
Agree that older Shimano XT or LX are top-notch - in fact some models of the old LX (M560 I think) have the same seals, cones and bearings as XT / Ultegra and the main difference being a Cro-Mo axle vs Steel. Don't go for fancy straight pull spokes etc. in your first attempt and pay extra for a good quality spoke key.