I'm thinking you could probably find something like that at Lee Valley or House of Tools etc. as far as woodworking goes. NAPA or Lordco probably have it for automotive use too - I'm not sure there'd be much difference. I've only ever used the stuff in a spray can myself though, but I haven't needed it very often. One thing that can help a shined-up belt that's slipping is to take it off and rough up the "V" surfaces with an angle grinder with a thin cutoff wheel, NOT a grinding wheel (a file/rasp might work too, but would BE more work as well ). It'll restore a rough, more grippy surface to the belt. I've done this numerous times at the sawmill I used to work at - it isn't always a permanent solution but can get you some more life out of a belt anyway.
If you are refering to wide leather belts post your question at OWWM.com. If you are refering to rubber 'V' belts or the small drive belt on a lunch box planer it is time for a new belt.