there are a couple of things that you can do.
1, self sign.
yeah that's the budget option, and requires you to say to people that they need to install your public key from your CA, but it's immediately free and ready to go after a couple of minutes installing openSSL
2, buy a cert.
I can't think that there would be many companies out there invested in a cert (that costs $200 a year) that will sign code (that is to say put their name on it to say it's 100% trustworthy, without having done an audit of said code.
realistically, if you find a free service that will sign your code, then it's likely that sooner or later someone will release malware code using that cert, and pretty soon the CA will revoke that cert and apps signed by them are not trustworthy...
You may find some cheap services:
they offer certs for open source projects for 14 bucks. (they did used to be free) there is some noise online that the tradeoff is that they append the CN Open source developer - "your name"
but then they offer reduced costs to promote open source.
I don't subscribe to the model that because you have a bunch of cash your software is somehow better than any other persons...