Tritium markers can't legally be sold in the US but it's usually easy to order them from the UK and elsewhere. At the moment there is a shortage because of the pandemic. Prices are up, certain sizes are unavailable or hard to find, etc. But there are still some out there. mixglo.com (UK) is a decent source.

The other annoying thing is tritium spheres haven't been manufactured in years. I have an old gadget that uses one. Its sphere is partly faded by now, and I can't replace it as far as I can tell.