My prefered brand of 2-pack filler is Easy 1 by Upol.
The best price I can find is Halfords (seriously), but that is a 3.5kg container (@£25). As a decorator, I would that would last me 4 months (depending on my jobs- some exteriors, I will use 3 of them in one go).
It should be noted that it is very rigid. It will (might) not accommodate movements in joints. For that you need epoxy resin fillers, they are much more expensive and often require special double barrelled guns and need to be mixed very, very thoroughly.
If you are just filling in dinks, most 2 pack fillers will suffice.
They harden very quickly (assuming that you follow the mixing ratios). Because they are relatively hard to sand back, fill, sand, dust off and fill again.
I use scrap bits of laminate flooring or plywood to mix it up on.