When I run out of my homemade curry paste, I have no hesitation about substituting jarred paste of a good brand. Here in Switzerland, Thai Kitchen is easy to get. If buy off the shelf, choose a brand that doesn't use chemical preservatives or mysterious ingredients. Thai Kitchen, for example, contains only water, garlic, red chiles, shallots, lemon grass, raw sugar, salt, fish sauce, galangal, kaffir lime leaves, coriander, cumin, and cardamom. I use about a half a jar at a time, and the rest keeps for quite a while in the refrigerator.