A/C O rings

Harbor Freight has a kit of HBNR O-rings (green). BTW, HBNR has also come in blue and red. The industry is trying to standardize on green, since has been typical in after-market. If R-12 or HC (Duracool, Enviro-safe), you can use Nitrile/Buna-N (black), but better to stay green just in case R-134A someday (to be discontinued) or the new HFO-1234yc (compatible?). Viton (brown?) is better than nitrile for most things, but haven't heard of using it in AC. I don't know what O-ring if they change to CO2, but that will be 2000 psi so wouldn't work for your system anyway.

Are you re-installing used hoses? If so, I would change the rubber. Any AC or hydraulic shop should be able to do so. I have hose and a Master-cool crimper, but you could even buy hose on ebay and Oeticker stepless crimps. You don't need the crimper pliers if you have a nail puller (even the cheap Chinese one). Can also use screw hose clamps (Breeze smooth ones are best) but the Oeticker crimps look better and can use 2 to be extra-safe. After-market AC in the 60's used screw hose clamps (my 65 Newport) and they didn't leak.