Your best bet to play Half-Life: Alyx as soon as possible is probably the Vive Pro. It's expensive, but it's actually in-stock. The Index is good if you don't mind waiting a few weeks, and Valve will let you place an order now. Everything else will look a bit worse and requires you to scour the web for resellers or eBay listings. PCMag: Best VR Headsets for 2020(Opens in a new window)
Half-Life: Alyx Review Roundup: Everyone Loves It
Half-Life: Alyx Dev Team Holds an AMA, Half-Life Games Currently Free on Steam