Mine are dim to not functioning. What are my options to replace them with? TIA.

I haven't heard of any replacement night sights for them.  Doesn't mean they don't exist, just haven't heard of them.  There are 2 sets of replacement sights I've herad good things about.  First is the Mojo Sights which I believe just replaces the rear sight with a peep sight.  The other is the Williams Fire Sight Which should replace the front and back with a peep sight on the back and a fiber optic sight on the front.  Heard excellent things about both.  If you want to keep the original sights and just replace the tritium part, I believe you are out of luck.