hi. to avoid bottoming out, I suggest you try increasing the compression damping first. that and landing better :-)
I got the spring from K-Tech in the UK, but its a "standard 20% stiffer XR600" spring so you'd get it via Baja, XROnly etc.
I didn't change the front springs, but changed the oil from 7W to 10W to improve rebound damping. you have to back the compression damping off to compensate, but it puts you mid-range of adjustment for MX tracks and low-end for everything else.
I just got back to the UK last week so I'm looking for a job!