What software do you use at the moment to listen to your music on your PC?... have you been able to check if it has a conversion mode to allow you to convert from WMA to MP3? If it does then you should be fine without finding another source.
Also, you don't mention if the phone is an Android or Apple iPhone... and it may also depend on the operating system you have on the PC? Win10?
I'd suggest doing a Google (or similar: Cortana if it's a Win10 OS)) search based on: WMA to MP3 conversion
to see what that brings up - quite a few unsurprisingly.
It's ages since I did any conversions for music files. I recall having to use a program called Easy Phone Tunes which ties in with iTunes to convert that way - with having an Android phone.
Sorry I can't be more helpful... good luck