Until recently, the only option for putting a 21" on the mid-glide Dyna front ends was to lace up a rim to your hub or have a wheel company custom build one for you. Now, American Classic Motors on eBay and J & P Cycles sell assembled wheels for your application at a cost of just under $500.00. These wheels have chrome hubs, chrome nipples, and chrome spokes, which require a lot of maintenance to keep them free of rust.
You can also go through a company such at hdwheels.com and have them build a wheel. These start at about $600.00 and go up from there if you want stainless components.
Lacing a rim to your hub is still the cheapest option. A chrome rim is available for about $100.00, Stainless nipples and spokes will run you about $125.00. The steps to lace the wheel are available in the service manual. Self lacing will save you a considerable amount of money. Truing the wheel can be done by the dealer for about $40.00.
I found a bent rim which I purchased for $50.00. It included the tire and rotor. I used its hub to build my wheel. I was able to sell the extra rotor, tire and my old 19" wheel assembly to help recoop some of the out-of-pocket costs. This allowed me to build my 21" wheel with a new tire, ready to install, for under $200.00.
If you purchase a wheel, make sure you get one with bearings for a 25mm axle.