I'm thinking on getting either 18's or 20's but need to know the proper backspace AND offset since rims for sale never show both. My preference is a 9 inch wheel

 

i think you meant backspacing- backspacing is the distance from the inside rear edge of the rim to mounting surface of the Hub (face down)
offset refers to the distance from the true center of the rim to the physical center of the rim (mounting surface of the Hub)... so on a 9" rim, if it lists 0 offset, then the backspacing is 4.5" lets say its 9" with -35mm offset, (which is 1.37") then the backspacing is almost 6" so- positive offset will widen the vehicle stance, negative offset will narrow the stance. when you read that a wheel is 9" wide (by manufacturer usually) they are measuring inside lip to inside lip- so, in all actuallity, that wheel is in the ballpark of 9.5" wide