Hey there,
I don’t have experience for sperm donation, but we went to the Czech Republic for egg donation. Friends of ours went to this clinic in the Czech Republic
https://www.reprogenesis.com/donor-program/donor-sperm/ as they needed sperm and egg donation. However I’m not sure if they’re donor programme is open to solo parents or not as I know some clinics restrict their treatment to couples :/
Good luck, I hope you find a good option!