Your Sun AT18 has a 393 ERD, which I believe makes it a 20x1.75 rim.
In 135mm spacing, I found one with QR skewer for less than $70 with shipping:
Bicycle Wheel Rear 20x1.75 406x19 Alloy Sl 36 Alloy Cassette 8/9sp Sl 135mm Ss2.
...and if you don't mind a nutted axle, right around $52 with shipping:
Bicycle Wheel Rear 20x1.75 406x19 Alloy Black 36 Alloy Fw 5/6/7sp Black 135mm 14
Everything else being equal, I'd take the QR over the nutted, unless you never plan on removing the rear wheel in the course of folding/packing.
These are the most cost effective options I can give you if your hub can't be replaced.