How do I know if a slipcover will fit my chair?

I have a chair - like a squishy arm chair but not recliner- that doesn't have a cover on it - it is just the base chair - like one you would get at IKEA with no fabric on it. I want to get a cover for it, but I am not sure where it was purchased, so how do I know if a cover will fit and where is the cheapest place to get one?