The best kydex sheaths are molded to the individual knife the sheath is intended for. So for a custom knife, the knife HAS to be sent in to the sheath maker. A mass produced knife is less critical, but keep in mind that even if he owned the same knife and molded it to that one, yours could be just slightly (but significantly) different due to slight variances in production runs. Or a change in the manufacturing process may result in a significantly different blade profile altogether, such as a thicker blade; shorter, longer, or completely different choil area; or a modified tip are just a few I can think of right off.

As for the tight fit, keep in mind that a tight fit is just one of the possible retaining features for the knife, which is why most Kydex sheath makers ask that the knife be sent to them. As I understand, this does loosen up some over time which is why a second, or even a third retaining option is usually added, depending on the intended use.

Another possible option, if you don't want to send your knife to the maker, is to order the materials and make your own. I've never tried this for myself but it looks fairly easy, and a search will pull up a LOT of resources. Here is just one example I got when I searched "DIY kydex knife sheath"

http://willowhavenoutdoor.com/general-survival/the-1-hour-custom-kydex-knife-sheath-in-your-kitchen/

As always, YMMV. Also, usual disclaimers apply.