I have a search set on old-school diagnostic tools on Offer-Up but I'm sure you can do the same with Craigslist/Facebook MP/Ebay. I picked up a couple of units for about 10 bucks each. One is really big and you need to set it down on a table or something to work but I like it because the gauge is really big so I can read it from a distance. The other is a small portable unit that I keep in the trunk in case I ever need it but I have only used it once to compare readings with the big unit and confirmed that they worked.