Just about anyplace, bike shop or etailer. Measure the length from the outside of the back of the fork, to the inside of the front. Or the width of the crown, less the front wall thickness. If the fork has a closed bottom, stick a pencil in the back hole eraser first, push it until it bottoms, make a mark measure from there to the end.

Don't forget to subtract 4mm for working clearance. If in doubt, buy a longer bolt, and fix it with a saw and file.