In the past I used 3-5mm cutting disks in my grinder for cutting sheet and bar stock; however since running across the 1mm slitting or cutting disk I can’t remember ever using the older, thicker ones at all.
The 1mm disks allow for a much finer, controlled cut that can be especially useful when trimming along the edges of a vambrace, greave or other straight edge. Held edge on brushed past rivet heads they are great for the fine control required for rivet removal.
There are a lot of cheap versions out there, but having tried them I would always recommend getting the superior (and often more expensive) versions that are out there – at about 50p to £1 each.
Beware though, you cannot be as aggressive with the slitting disk as you can the larger cutting disks and it is true that they do not last as long as their thicker more sturdy cousins but the speed that they can cut and the finer control they offer, in my opinion, by far out weighs this point.
Normally I would expect to buy them in tins of ten from most decent hardware and DIY stores.
I use the Rhodius slitting disks simply because the local hardware store (Elliots) supplies them and I have found them to be great value. I bought some cheaper ones from a car boot sale and managed to cut about 12 inches of 16g mild steel before the disk was used up.
Keep the older, worn out disks as they allow you to cut tighter corners.