When I worked for Intec we ran custom XPages courses. But you have to bear in mind creating content for a course takes a lot of time (and skill, to build content that pitches at the right level). So it will cost.
But I’ll also echo comments above, that it depends what kind of development you want. Support and minor tweaks to Notes Client applications requires different training to XPages. Notes Client has overlap with developing for Nomad Mobile in terms of skills, but you need to consider the screen size, so the way you structure the application should change. If it’s support for traditional Domino web, that’s again something very different. But if you want to deploy modern web applications that interact with Domino via Domino REST API, the learning you need should largely be non-Domino-specific.
OpenNTF’s latest webinar covered basics of Notes Client development. There are various tutorials in Domino REST API documentation. I also wrote a blog series about taking an XPages application and converting it to use Domino REST API and framework-less web standards. I wrote a tutorial for Nomad Web before joining HCL.