Well beside the well known advertising networks they are numerous others out there but be careful many of them are scamers or are used by scamy publishers.
How do they scam:
They have a bot who is visiting your site through several Proxies. This causes you a significant financial loss I know networks which traffic is up to 50% faked.
The only way to check the quality of purchased traffic is to use javascript monitors for user experience measuring. They are usually made to check the scroll and click behaviour of users and the most bots does not click or scroll. They are some out there which do both but they click and scroll randomly and stays always on your page (so if you have external links they will NEVER be visited).
How to identify:
Ask me or google for every network with the words "scam", "fake", "opinions" maybe you could find videos by their victims.
Conclusion:
If you wanna buy traffic, use large networks but this would cause significant costs because even the big guys are not 100% save against faking. The second reason are their high CPCs due the higher competition between their high number of advertisers.
If you want to place advertisements successfully you have to know your conversion rate. A conversion rate of 1:50 and an average sale margin of 50$ would allow you to spend up to 1$ per click (taxes are not included).