I have a brown leather sofa and a beige fabric oversized chair/ottoman. What type/color slipcover should I get for the fabric chair so that they'll match?
Brown leather is neutral so literally anything will go with it.
Often it's best to not even try to match because you know you'll never find a color that matches perfectly. Much easier and ends up looking better to instead select something that will simply complimentary to your leather sofa. What that will be depends a lot on the style of your room and the other colors you are using in it.