If you're going to replace them, buy them as a flow matched set. otherwise you may be no better off, or you'll be worse off than what you took out. Flow matched sets cost a few bucks more but if you're going through the cost of replacing them for "maintenance" purposes, may as well make it worth your while. They're usually offered by level of matching as in how close to each other they are. The lower the number the better but that also means the higher price. 3% is usually minimum and lowest cost, 2% is mid-range and midrange cost, 1% is best but highest cost.

Where ever you buy them from should be able to provide a certificate of calibration or flow matching that not only gives the spec but also the type of equipment used. if they don't do that, keep looking.