I can recommend a great option
https://digitaladsystems.com/cpa.html, it is a software for CPA affiliate marketing, a comprehensive system of the highest quality, specially designed for buying advertising space with a pay-per-action model. CPA is a pay per conversion model, i.e. price per action. Unlike CPC (Cost Per Click) and CPM (Cost Per Impression) models, CPA allows an advertiser to pay only for a specific action performed by a user. This can be a product purchase, registration, subscription, app download. This product is perfect for custom ad networks, business lines, large content publishers and advertisers, I highly recommend checking it out!