Pressurized co2 really, really rocks but if you don't want to use that, or diy, or excel, then there really isn't anything else to use other than that. There os this electric block thing you can use to supplement carbon but it does it by eating up your KH. It's expensive, barely works and no one uses it... No one.
It doesn't get much easier than adding a few ML's of Excel or metricide (gluteraldehyde) if you don't want to use gas. Why so insistent on having a solid source? Alka-seltzer won't work ; p