There is ossign.org, Certum offers a cheap certificate for FOSS [1], and Comodo offers relatively cheap (but still expensive) certs as well [2]. Not affiliated with either service, but these are the ones I remember last time I had to dig into this mess, so there might be even more services that I don't recall at the moment.
reply[1] https://shop.certum.eu/open-source-code-signing.html
[2] https://comodosslstore.com/code-signing/comodo-individual-co...