Generic replacement knobs don't fit well. Too loose, or too tight. The ones that stay on are too large in diameter, and cover the lettering. Most junkyards are devoid of CJs/ YJs let alone ones with a/c, knobs intact. The original manufacturer actually still stocks the knobs, vent registers, all the way up to complete retrofit kits.
Search for jeepair.com. p/n 119-9896. $8.99 for both knobs
'91 YJ Sahara 4.0L/AX-15/NP231, D30/D35 @3.07
'12 KK Cherokee 3.7L/42RLE/MP1522, D30M/8.25 @3.73
"Non vi sed, Arte"