At one time I did a lot of 10mm conversions on 40cal. guns, most were Para P-16's.
I say that as that will most likely be your least expensive route if you can't find an existing 10mm.
If you decide to build one from a 45acp frame keep in mind that you will need to buy all the parts it takes to build a complete top end.
Here's a parts list of what you will need.
Slide 40 caliber
Barrel 10mm caliber (I would buy a ramped barrel)
Barrel Link & Pin
Barrel Bushing
Firing Pin ( most 40 slides use a 38 super firing pin)
Extractor (you should use a 40 caliber extractor)
Firing Pin Spring
Firing Pin Stop (I would use a small radius firing pin stop)
Recoil Spring (with guide and plug or FLGR and plug)
Front and Rear Sights
Ejector
Mags (however I've had good luck feeding 10mm out of 45acp mags)
I would buy a barrel with a Para style ramp as it's the strongest ramp style and the frame is less likely to develope stress cracks using the Para style ramp.
Good luck with your build if that's the route you decide to take.
Regards
Bob Hunter
www.huntercustoms.com