gear head for 5ft axis

Hi All,
I've being thinking to build a 5 axis gantry,'it would rotate a
headstock for A and B axis, where can I find zero backlash gear heads
for the 4th and 5th axis? what kind should I go for it?
There is split worm gear box, harmonic type, etc...
money is short as always, so the cheaper the better.
Also there is someone on the list that has built one?

Thank's
Fogassa