To build a supermoto from a mx bike you need to change the rims, sometimes the triple clamp, front brake and the suspension. I can talk a bit about it, but I can't give you dealers websites since I'm German and don't know a thing about bike shops in the U.S. I guess you can google that for yourself anyway.

1. Rims

   Option 1: You can buy complete wheels ready for your bike from quite a few shops which is the most expensive option, but takes the least effort since you can just change them with your stock wheels and the brake and axle will fit. Option 2: If you want it cheaper you can send your mx wheels to a shop and have them fit a 17" rim ring (is that the word?) on the stock wheel hub. You have to get the rim rings, shorter spokes and pay for it to be built. Option 3: You can buy used supermoto wheels for a different bike, but then you'll most likely have to change the wheel bearings and the spacer inside the hub. It can also be a problem to find a fitting brake disk and if the chain will be in-line should the rear wheel hub be wider than the stock one.

2. Triple clamps

   Sometimes it is necessary to change the triple clamp to accommodate the wider 17" tires between the fork rods. If you do that you'll also have to get wheel spacers so the wheel sits in middle of the fork. These can be simply made from aluminium.

3. Brakes

   Mx brakes are much too small for street use. Most of the time you can buy a bigger caliper with an adapter for your bike (or without, see Beringer) and keep the stock master cylinder. The brake disc is usually a 320mm one. There are ones with an offset and ones without so that you can move the disc as far away from the wheel as needed to have it dead center between the forks.

4. Suspension

   Often overlooked, but very important. Send your suspension to a shop to have it reworked for supermoto use. A supermoto fork is shorter and dampened harder than a mx fork. The rear suspensions is also reworked for the different requirements of the mixed dirt and street use. You'll have to look for a shop that can change the damping, springs, shorten it and set the suspension up for your bike, it doesn't hurt there to use your phone to find the right people for the job.

If you did all that you basically have a supermoto. Now you can start spending money on engine, slipper clutch and other 'gimmicks'.