I was diagnosed with HPV yesterday. How can I find who gave it to me?

May never know. Unless you have only had one sexual partner, you may never know. Not all docs test all patients all the time for hpv. The hpv test may only have been done this time in response to an atypical pap smear. All patients with hpv do not test positive all the time. The virus may have been present for years and was just now discovered and identified.Your partner may not know he/she has it.
Inform all partners. Inform all of your sexual partners of the past several years and inquire about the presence of visable venereal warts. If the number of partners is too numerous to count, then self circumspection(being more discrete and limiting partners) should be employed going forward. A good question to ask yourself, do I know this person well enough to share bodily fluids?