Hy.
I try to this, reselling seo services but I can not say it was a nice experience, maybe this not fit for me but you can make some bucks if you do it right.
Now what I can suggest is first to find a good seller that offer good services and start a discussion with him to agree with payments and prices.
Second I will ask him where he sell his services and try to find some unexploited marketplaces, see if your service fit there and most important if your price are competitive there.
About a source where you can get them... Here I can't give you any suggestions since I was selling services from different forums. Here you may find some trusted seller with good services and I think you can get to an agreement with them.
Thank you and good luck.