Leopold spacebars can be replaced (I'm typing from a Leopold with a custom blue spacebar right now), but they have a non-standard spacing for their stabilizer mounts, so the more common Filco style spacebars won't work unless you want to actually *mod* the keycap, which some people on here have done. Leopolds are relatively new on the scene and are fairly popular, so hopefully we'll be seeing more custom Leopold spacebars in the future.
Rosewill is a pretty safe choice if you're looking for a full-sized keyboard. They do have the same OEM as the Filco (Costar) and are significantly cheaper.
The Ducky 1008 is supposedly very poorly made, but the Ducky 9008 is higher quality and should be more in line with the Rosewill and Leopold.
All of those keyboards would work with white PBT keys, but be aware that PBT Cherry MX spacebars are not particularly common. They are out there, but you might have to do a bit of hunting.