Where to purchase single commercial debts?

Debtx. You could also try to contact individual debt firms like CFG Partners.

This will sound pedantic: you should be careful around small/medium business debt. The representations were bullshit before COVID. Bankruptcy sales are safer, but also understand that the value is whatever the court says it is. Idk if you're refinancing distressed borrowers, but if you are instead purchasing a preexisting claim and substituting yourself in before a plan is confirmed, then you want to take a proactive approach with the trustee. I've had an instance where an asset was removed from the bankruptcy estate because my firm and another were the only creditors with claims to the asset and we both agreed to move in for a sale. Months went by without us receiving proceeds from the sale so I contacted the trustee to ask about them. The trustee tried to get us to agree to a cramdown despite there being enough proceeds to make us whole because the other creditor agreed to a cramdown. 

Another piece of unsolicited advice. I'm going to sound like an asshole, you need to be a disciplinarian with distressed debtors. Too much flexibility (too much mercy) will smack you in the face quickly.