check the for sale section. you might be able to get a cheap used factory rear pair. :)

You definitely want to match your tires. Mixing and matching front to rear will yield all sorts of handling issues if the compounds are different enough.

replace the tires buddy. the treads need to match, and every brand makes their tires a little different

factory rear pair = 245/45/18 right?

Yep

the size can be right all day long, but they should/need to be the same brand/style so the tread patterns match