Call your local bottled gas place about a CO2 bottle. They generally have used ones for sale. they’d probably sell you a regulator too, but I think NB’s economy regulator is about as cheap as it gets. You really don’t need a gauge for telling how much is in the gas bottle because the stuff is liquid and until you are about out of liquid the gauge really doens’t go down. You just need the gauge telling the psi output.
Then call up local soda distributors and see if they sell cornie kegs, I used to get them for $20 but the price is up to $30 now.
I bought a tank off ebay and darned if the cert wasn’t almost up, they guy knocked off a few bucks when I complained. They need to be done every five years and it costs something like $10, so ask about the date.