Finding out you bought the wrong kind of circuit breakers as back-up when you have a power outage is no fun. Whether you just bought a house or had one built, you need to find out which kind you have and if that is the only kind that will work. If you are buying an older house, you may find out the type is obsolete or very hard to find. You should buy a complete set if you can find them. If not, you may want to think about having them changed by an electrician.
One of the more popular brands are Cutler Hammer circuit breakers. You may need to find a refurbished set if your model is older and no longer being manufactured. They should not be too hard to find at an electrical supply outlet. They may even be able to refurbish yours if they do not have the right model in inventory. You may also be able to find pre-owned ones from a contractor that has been in business for a while. Many contractor companies will keep things like that is stock for future jobs.
Some of the other more common brands include Federal Pacific, American, Westinghouse, and FPE breakers. You should not have trouble finding replacements for these brands, even if they are older models. They were so widely used, someone will still have them around. I still suggest buying more than you need for the future though, just in case. You would not want to have to have major electric work done if you are not planning on it.