Rectifier tubes?

I have an Air Tight HTM-300 amp which has tube rectification. I've rolled a number of NOS types over the last three years and am wondering two things: 1) How long a life do these tubes have, in general, and 2) Any suggestions for good rec. tubes (5U4GB) in this amp or in general? I'd especially be interested in new tubes as NOS are hard to get and are iffy.