From a non-military standpoint (I am a retired Army medic), your best shot at getting prescription fluids, tubing, and catheters is to work in the field--EMT/whatever.

After that, you might be able to convince a sympathetic physician to write you a script for those items--but good luck. Also note that in the latter case, you could be held legally liable if you used your rig on others.

In twenty years, I've only had two cases (out of thousands) of dehydration that warranted an IV--that being said, my medics stuck 'em all anyway for practice.