Found some - and they should be here early next week.
5/8-11 is correct. I checked Grainger and other places, but ended up doing an internet search and ended up at castercity.com
http://shop.castercity.com/ProductDetails.asp?ProductCode=K3APB4BX-MLI called and spoke with someone who recommended the above due to the weight and that it would be stationary 99.9% of the time. He recommended polyolefin and a total lock brake that locked the wheel and swivel all together. Put the 1 3/4" stem on the 4" wheel which has a 5" overall height, with stem at total of 6 3/4". I have the sidewinder 5" off the ground (to botton of leg and could go a bit lower right now). With a 15% discount code for calling in, shipping, it's just around $90 for the 4 shipped.
I'll keep you updated when I get them in.
Now - I need 4 more sleeve platens and (6) 10-12" youth platens. Trying to find a machine shop to cut some 6061 aluminum to save some money.....unless someone has some for sale cheap.
Thanks!