you can cut the inner rib of the zx5 front wheels, or use ultima RB5 front wheels, or cut the inner rib of zx5 sp front wheels. Each option gives you different front widths.
The offset of the zx5 front wheels is a little bigger than on the zx5 rear wheels.
If you like spoke wheels, kyosho also has some. I think they are priced the same as the ZX5 wheels.
Personally I'd go for these:
http://www3.towerhobbies.com/cgi-bin...3&I=LXNX94&P=K
and
http://www3.towerhobbies.com/cgi-bin...3&I=LXNX95&P=K
They're a nice match, with wide offset in the rear just like the original ultima.
PS: yes the RB5 uses 12mm hexes, hpi also, traxxas also, but the rulux ones I don't think so.