True. Almost all 14mm axled 20" wheels will not have a disc mount. 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.
The 2nd option is to do it properly and build your own wheels. As Paul says 20mm is overkill and only needed if being exceptionally aggressive off-road. Sturmey do a single sided hub with a 12mm axle and that is the size hub I'd look for. These would be fine and use a centrelock disc.
The Shimano HB-RS470 front disc brake hub features a Center Lock design, Internal Grease Sleeve, and Cup and Cone Bearing technology. <ul> <li>The Center Lock system makes an easy rotor installation possible with spline mount and a lock ring</li> <li>Hub features internal o-rings and a...
www.modernbike.com
There are other similar ones with 36 spokes but they get a lot more expensive as they also move up the quality scale.
If you insist on 20mm then:-
Smarter Shopping, Better Living! Aliexpress.com
www.aliexpress.com
and these use a conventional disc mount. Be aware 20mm bolts cost a lot more than 12mm and weigh a LOT more too.
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 going to work for 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 cheap BMX wheels. They won't take single sided mounting.