The model 10 dials were built into certain Western Electric branded Trimline rotary telephones and can be sourced from ebay, although it doesn't seem possible to determine by looking at the outside of the phone whether it contains the correct dial. Oldphoneworks.com sells the dial by itself here, although there are sub-variants which will either not work or wil require more extensive modification, so even then there's no guarantee that the first dial you acquire will be suitable.
Why was this specific dial chosen? They're more compact than the standard Bell System rotary dials because they were designed specifically to fit within the handset (not the base) of the early rotary-versions of Trimline telephones. A clever feature that allowed the use of a smaller diameter dial plate is that the 10 finger holes are almost evenly spaced around the dial, with no "dead space" between the 1 and the 0 to leave room for the finger stop. At first glance it doesn't appear possible to dial a 1, but when you start dialing you find that the finger stop moves to roughly the 6 O'Clock position, making space by allowing a kind of overlap in the 1 position with the 0 position.
Just any old Trimline dial won't work for the rotary cell phone, though. Trimlines were made by several companies (Bell had a lot of control, remember), and specific ones made by Western Electric had more compact mechanisms behind the dial. It was important to me that this phone be as small I could make it, so this is what is needed.
The models found to be usable to date are Western Electric models 10A, SC10A, and maybe 10D with a lot of work. Nodels 10QA, 10G, and SC10A look close, but they're missing the two 4-40 terminal screws near the reed switch that allows the dial to mount to the cell phone main board (the normal mounting tabs get cut off to make room for the cell phone's buttons). Model 10D may be usable, but with a lot more work in modifying it. Unfortunately, even the model 10A has subvariants that don't include these terminal screws, hence the importance of the below information:
With the cover out of the way, the important features to look out for are the presence of two screw terminals and that the fixed end of the reed switch be "potted" instead of molded plastic. This is highlighted in Figure 1:
Figure 1: Model 10A with
terminal screws and potted reed switch Some reed switches may have one reed electrically connected to its screw terminal via a metal bracket and the other connected by a wire with a lug, as is the case here. Others may have both reeds connected by wire. Both are fine.
Note that these dials often come with a plastic cover of some kind over the clockwork, and further note that they vary significantly in color and opacity, so don't get thrown off by that. A common and typical cover is shown in Figure 2, but you might also come across something like what's shown in Figure 3:
Figure 2: Model 10A with
clear cover and non-potted reed switch. 
Figure 3: Model 10QA with
whitish cover and potted reed switch. Another good dial is SC10A, shown in Figure 4. Note the encapsulating plastic piece that's left on the potting for the reed switch, which should be fine.
Figure 4: Model SC10A with different potting style. There are many dials that look close but won't work, or rather, would require one make a custom enclosure to mount the dial differently. Note the lack of screw terminals on the dials pictured below:
Figure 5: Model 10G (bad). 
Figure 6: A weird Model 10A (bad, shun it). 
Figure 7: Model 10QA (no good).