Inexpensive router that spoofs MAC address?

> [...] My cable modem already has wi-fi and anything else I need. [...]

   What, exactly, is your "My cable modem"?  The fact that it "already
has wi-fi" suggests that it is a modem+router, not a simple modem.
Knowing nothing about it makes it tough to determine whether it already
lets you spoof its MAC address.

> [...] I need to purchase a router that can "spoof a MAC address",
> [...]

   Because you already have a router in your (unspecified) modem+router,
adding another router might make very little sense.  Especially if what
you already have can do what you want done.

> [...] in order to change my computer's IP [...]

   Strictly speaking, a different MAC address for your router might
cause your ISP to change your router's WAN/Internet IP address, not your
computer's IP address.  But your router's WAN/Internet IP address is

what's visible to the outside world.