We try internal coverage first. Then directly with local docs...usually staff from the local military hospital or fellows at the university. They get paid by the same formula as we do.
Try to go through gaswork or ask around through local word of mouth. All I can say is agencies charge a pretty penny and don't add much value in spite of all the promises. If you can get someone in that won't upset the applecart (ie: your more demanding surgeons) and is safe it is definitely worth paying a premium over your partner hourly rate.
It's amazing how everyone needs help at the same time. New grads should be demanding signing bonuses and shorter partnership tracks. People are retiring into this booming market/covid environment left and right. But it can all change overnight.