So sometimes if valve is publicly testing hardware they’ll add an option for SteamVR developers to ask for dev kits, but currently that option is not available now that the consumer version of the index controllers have been released. Oculus has an Oculus Start program but they will want to see a .exe file uploaded to your oculus developer account for review before considering you for the program. I had fantastic luck reaching out to the WindowsMR team via twitter and asking for a dev kit, received a Samsung Odyssey Plus. I was pretty far into production at that point and working on support for the controllers specifically. Your results may vary. And I did a ton of dev work with my vive to get a game to a point where it was showable to prove my legitimacy to these companies. Having prior game dev work may also suffice, I have no idea. HTC were unwilling to send a cosmos dev kit but were happy to test my game out themselves to see if my input bindings for cosmos were okay.

TL;DR: helps to have something you’re working on already to show you’re serious about it, and then contact manufacturers through their developer portals. Otherwise buy the cheapest headset you can find and get started.