I'm converting my mill to ballscrews, I'm looking for 1/2" dia rod and can only fit a 3/4"dia x 3/4-1" lenght. Should I get a pre-load ball nut or will a reg ball nut be just fine?

A preloaded ball nut is not "draggy" as the term "preload" might imply. I haven't had much experience with commercial "rolled thread ballscrews", but I would tend to think that they are assembled without much care or testing of the clearance when fitting the nut to the screw.

Do you know if the ballscrew and nut you are looking at is hardened? If not, it may be possible to "wear it in" by repacking it with larger balls.

It is possible to buy balls that only differ by steps of .0001 (or maybe even less) from the nominal size. A lot of smaller ballscrews are based on a 1/8" ball. The cheap ballscrew and nut is likely just "thrown together" with balls that are of a size that is sure to fit without jamming up.

So, you could buy the ballscrew and then go elsewhere and buy some oversize balls to try to reduce excessive clearance of the assembly down to what you can tolerate: .001 or .002" backlash is not desirable, but you can live with it if your tolerances will allow it. That is to say, you can quite successfully climb mill with that much backlash.

One trick I've learned about packing my own ballscrews, is that only every other ball should be a close fitting one, and they should be spaced apart by an undersize ball. This is so the undersize ball can roll freely between the two balls that are in "firm contact" with the raceways. This prevents the balls from jamming up due to the counter-rotation that occurs when one ball catches up with the one in front of it.

Who has the best prices on them?

I don't know about best prices but you can purchase precision balls from:
Bal-Tec Div
1550 E> Slauson Ave.
Los Angeles, CA
1-800-322-5832

I have purchased rolled ball screws and nuts from Roton for a cnc mill I’m building. Roton will preload the balls to any “tightness” you want. Good customer service and good prices. They actually return phone calls.
http://www.roton.com/index.jsp