Try Northern Tool. It looks like a regular plastic wheel with a tire and tube. It looks like it is held on with bent into position aluminum cheek plates. Simple.
I have the dual roller blade wheels on my Bensen. They work great, cheap, you can get different hardness' plus the come in HOT COLORS!
Some poor kid in the neighborhood is missing 2 wheeels off his skates!
I also use the rollerblade wheels on my single place Air Command. They work great, except when you try to push your gyro backwards on grass. Easy to find, and cheap to boot!
I have the scooter wheels on my Air Command....they are larger than the skate wheels. The scooter wheels are approx 100mm where the skate wheels range from 72-80mm.
They work really well and should last a long time if you compare the use they get on the gyro to the miles they are ridden on scooters and skates.
Mike Gaspard put the skate wheels on his Benson as it requires a little smaller wheel unless you modify the bracket.