You could start by asking your LDS. Many sell off rental gear. If there's a particular kit you like, ask if it's available. Quite a few of the LDS at the swap are selling of rental gear. Generally nothing wrong with it, they just try and replace periodically (just like any rental company).
It's a risky venture for newish divers to buy used gear. Generally have not learned enough about it to know what's serviceable and what's not. I met a guy last summer bought a used Dacor reg set, only to discover that service kits for it are no longer made/sold (Dacor was bought out). There had been a trade in/up program, but it had also ended. In the end, instead of saving $$ he had a rather expensive set of guages (they were useable).
