Berkshire Hathaway is a textile mill that Buffett turned into an investment company, but by then he had been a successful investor for over 10 years.
It's hard to lump financials in with tech startups, though. Technically, when he Buffett started the Buffett partnership, he had lots of cofounders. The structure of an investment fund is a single managing partner (who makes the decisions) and then lots of limited partners (who put up the money). In the eyes of the law, they're all cofounders.
If you go by practical instead of legal definitions - who provided the know-how? - then Buffett's early partner was effectively Benjamin Graham. After all, it was at Benjamin Graham's firm that Buffett first learned about investing, and Graham served as a mentor long afterwards.