I have dollies for all of my implements. For most of them, including a 4-foot box blade, I use the 2-inch steel-wheel castors which I buy at Tractor Supply for about $5 apiece. I like to use steel wheels because they stand up to the grit on the floor and they don't develop a flat spot on one side if they stand in one place for an extended period. As long as I sweep or blow off the cement floor once in a while, they work great. However, when it came to building dollies for my chipper and my disc harrow, I bought 4-inch steel-wheel castors from Northern Tool for about $14 apiece. Each of these implements is very heavy and the small castors won't swivel well under that much weight. I expect these steel-wheel castors to last forever and these dollies have certainly improved the state of my shoulders and back. Actually, it's kind of fun designing, building and refining these things. It's not rocket science.
The attached shows how I store my attachments. Whenever I need one I just pull it out of the line, turn it sideways and attach it to my tractor. Each pallet is 2' x 5'. The frame is made from 2"x6"s and the top is sheathed is chipboard. I have fixed 2.5" plastic casters on one end and 2.5" swivel plastic casters on the other end.
Mine aren't nearly as neat as those pictured, but do serve the purpose. I purchased car dollies (each holds 1000 lbs) for about $30 (2 in a set) and put boards across them. This works really well for my 72" Box Blade, Ballast Box, and other implements. They roll very nicely across the concrete floor in my polebarn and sure make the attachment process easy.
Get casters that have so-called double ball bearings on the swivels. The less expensive casters have ball bearings between the base plate and wheel, but the post that hold the wheel to the base is just metal to metal on the opposite side. This causes the wheels to jamb and resist swivelling when loaded down and it's very annoying. The better casters have two sets of ball bearings,on on the top of the wheel between the frame, and the other under the wheel frame where the post comes through. These swivel MUCH better under load.
I started out buying movers dollies form Home Depot and they are complete pieces of ##%#$. The swivels constantly jamb and it's impossible to steer the implement when rolling it.
I subsequently bought the more expensive casters (almost $20 each) but it's really worth it for heavy implements.