My supply house told me the High Output 110 watt t-12's will still be manufactured for some time as there needed in cold weather installations. As stated before, the T-8 tandem fixtures put out better light (lumen/watt) then the T-12's ever did.
There are retrofit kits available to convert t-12 into t-8 lights...but why bother. For $40 i can get a new tandem t8 fixture plus $6 for a set of lamps.
Personally ive stockpiled a bunch of t-12 HO lamps and incandescent 100, 75,60 & 40 watt lamps. Then ill slowly switch over to led LAMPS AFTER THE PRICES DROP ... not when they try to force me to. Also, if you have 65watt recessed lights with incandescent lamps....best stockpile lamps. there on the way out.