That depends on the city. If it was originally a company town, probably the company. If it's a new build, probably the property developer. But either way, something that already exists is not subsidising the plumber's salary. The plumber is paid by the customer.
Anything built post-GD (which is going to be the vast majority of what you service) was likely touched by government subsidy in some fashion, whether grants, loans, or municipal bonds (tax-exempt, so effectively a subsidy). "Company towns" built before then were often subsidized, in effect, by companies not having to bear the financial burden of their many, many instances of illegal or rights-infringing behavior.
That's not considering your customers, who are likely also subsidized by the government in some fashion - if their jobs do not involve federal or state government contracts or supplying or servicing companies holding such contracts, they're almost certainly taking advantage of advantageous tax rebates or deductions.
You could equally say that tax money is all private money, so really it's all private. It's silly to talk about the provenance of money in these topics.
As in if I am a private plumber who does jobs for people, my job is subsidised by taxes?