The baking soda/acetic acid method is practical if you can rig up a tray of baking soda and a very controlled slow drip onto it from a bottle of vinegar.

Yeast, sugar, and water also make CO2 during the fermentation process.

Seltzer water should leaved no residue- you were probably using tonic water or something with sugar in it- but is not a good idea, as you can indeed leach nutes out of the leaves if you overdo it.

Those are the short-money options... but the ECONOMICAL option is a proper CO2 tank and regulator in a sealed room. Additional CO2 is nearly worthless if you don't already have your technique and grow area optimized.