The various big home goods/stationary/art supply chains carry small plastic three-drawer cabinets and the drawers make excellent trays for up to about 8x10, the whole rig is 10 or 15 $'s. Litter trays and similar can work well for larger sizes. Plant starting trays work nicely for tray processing 7x17 film, but you need at least two of them together to get enough stiffness for rocking. Lots of stuff will work, and work well, though do avoid aluminum because it's reactive with much of our chemistry.
Or else the used stuff is available from sources like ebay and, your local Craigslist probably has at least one ad offering darkroom stuff, though I've found the CL listings to be highly variable from jaw dropping bargains to breath-taking hutzpah.
I've never had an issue with contamination from old trays, just give them a good washing and liberal rinsing.
Also, as someone stated, if you're planning to get into this for a while, get the hurt over with and buy some new stuff from one of the usual suspect sources. Clean, matching and purpose made trays aren't essential, but they make life easier and the investment can psych out your head into elevating your game.