Almost all 14mm axled 20" wheels will not have a disc mount. They do exist and are just slightly more common than unicorns. Three options:-
Use 14mm axled BMX wheels and bodge the disc mount by getting wheels with a thread for screw on disc adapter. You need to bolt the disc adapter through the hub as the left wheel will simply unscrew the adapter on braking. I also epoxied the adapter on as well as bolting it. You'll need wheels with enough hub flange to allow you to do this. This bodge worked just fine even under stoppie inducing braking. The epoxy is the main fix with the bolts being the back-up.
The 2nd option is to do it properly and build your own wheels. 20mm is overkill and only needed if being exceptionally aggressive off-road. Be aware 20mm bolts cost a lot more than 12mm and weigh a LOT more too. 12mm hubs will be fine with ht bolts as axles. Shimano do some cheap centrelock ones and a 6 bolt adapter works out cheaper than centrelock discs.
You could also look for a 15mm thru axle hub and get a 15mm to 12mm sleeve to enable you to use a 12mm bolt. 15mm bolts don't exist or if they do are rocking horse poo. The 15mm alloy "bolt" that normally goes in them is only going to work if held at both ends and that isn't up to single sided mounting.
The 3rd option is to use sturmey single sided hubs with built in drum brakes. Nice but very expensive.
Do make sure that if you make your own wheels you buy rims with the same number of holes as the hubs. There are spoke length calculators on the net to enable you to buy the right size and lacing wheels is a doddle. It looks complex but it's not.
Do NOT use 3/8 axles which form the majority of 20" wheels. They won't take single sided mounting stresses.
There is a left field option which is a couple of front wheels and hubs from a mobike. They are single sided alloys on drum hubs but 24" and solid tyres. No idea on availability outside the UK.