There is a ton of providers everywhere, but they are split into many areas.
JAP and some other panels provide their own 'exclusive' services, but most of the panels are reselling the same services from the same providers.
The main providers don't have websites or public profiles, you need to find them, or get so big that they contact you directly. (avoid fake "providers" on telegram or so, they are almost all scammers)
You can became a service provider yourself, but keep in mind that is not as easy as it looks like, for 'fake' services, you need a ton of accounts to warm up, churn, and burn, a big system to provide reliable services, and preferably automatically.
If you search enough, you will find great services that are real advertising services (jap (re)sell some great such services), but it's more expensive obviously.
Finally, before doing anything, you need to try the services you want to resell, and expect replacing services regularly, because some services stops to work, you don't want to resell broken services, right ?
Also, JAP is cheaper for certain services, because they have a huge volume, so they can negotiate lower prices.
But your question is a question that is asked regularly here, the reply is always the same: there is no "main provider".
