I found these rubber caster wheels at Harbor Freight Tools for $3.99 on sale.
They required a bit of modificaiton to use with the skateboard.
I had to cut about 1/8" off the metal tube spacer to make them work,
and also use a large washer to hold everything in.
They have a plastic bearing cap, which wont work, so I used the washer instead.
They have their own bearings, which are prob not as good as ABEC bearings,
and also they are heavy (946grams vs. 270grams for Urethane stock wheels).
The ride is a lot smooother and more stable though.
Now, if I could only find 5" diam inflatable wheels with rims.