Hi Dom,
It's not common to use ball/roller bearings but I do fit them. I use drawn cup needle roller bearings which run on unhardened silver steel axles. I believe Neville Evans? used this method as well. I expect the axles and bearings to outlive me and probably the next owner as well!
You can get the larger bearings with built in seals but for smaller ones you have to add your own e.g. O rings either side of the bearings.
Arc Euro sell metric bearings and imperial down to 7/16" bore but any of the bearing suppliers will have them.
As Clive suggests though, you may have trouble fitting the bearings into the standard axle boxes – the boxes may be too narrow to take them, although the drawn cup bearings are quite thin, unlike standard ball or roller bearings, and there is no need to use a hardened inner sleeve.
My 2½" gauge Helen Long has 10mm bearings on all driving axles and 8mm on all bogie axles and is very free running. The 0-6-2 Simplex I am building also has Drawn cup bearings on all the axles.
For axles near or under the firebox I make the axles fully enclosed in a tube (like a cannon box) to keep all the grit etc. out.

John
Edited By John Baguley on 19/02/2014 10:46:07
Edited By John Baguley on 19/02/2014 10:47:34