Old (and not so old) printers and scanners may be a supply for those things..
Timing belts used for moving printing head (30cm in dia) could be found in them.
Then, you only need a small pulley on encoder shaft.
The big wheel (or pulley) does not have to have all the teeth, only couple of them are enough (say, every 50th or 100th tooth is enough to keep sync and to prevent micro-slippage). Those teeth could be made by gluing them on the perimeter of the big pulley (appropriately dimensioned of course), using timing belt as template
In this way, it is easy to obtain 10:1 or more up-gearing (tested, the concept works
)