Just buy a can of Walmart's $1.99 black, cheap paint. You don't do a full rattle can paint job on a car when guide coating, just a light mist on the panel. That can will last a car or two (or three).
Here's an OK video
I have probably 1/2-3/4s of my can of SEM guide coat which according to the price tag was only about four bucks - and it did my entire '31