Finding an SSD that's compatible with a late 2007 MacBook?

I upgraded a similar era Macbook a while ago. In all likelihood you are unlikely to be able to buy an SSD that will not max out the BUS speed available to it on a machine that old.

That particular Macbook has a SATA I (1.5Gbps) interface for it's existing hard drive. SATA III (6Gbps) SSD drives have been the standard for many years now, although you may be able to get a bargain bucket SATA II device (3Gbps). Basically, I would get the cheapest SSD you can for the capacity you require that is at least SATA II (you don't need it for the speed, but a SATA I one will be based on older technology that may have a shorter lifespan than a newer drive). It is almost certain to be faster than your machine can handle, so there is no point getting a particularly recent or fast one.