They normally come with the radiator but your local DIY sheds may well stock them. But the main action is the tightening with the spanner. I don't see the caps as being that important.
As GG's post indicates, they're known as 'decorator's caps' (or 'commisioning caps'). Screwfix is as good as anywhere on the High Street if you don't mind having to buy a pack of 5.
Local hardware stores might sell them singly though. (I'd bet that the one down the road from me does. They even sell things like tiny cable ties singly, for just 2p each!).
Drain your whole system, when finished doing whatever flush it all through and replenish the inhibitor.
That way you will avoid swarf in the system.
If you are just looking to paint your rad in an off site position buy the caps as described above. Personally I would still go for a drain and flush as you will undoubtedly fins a ton of back mucj comes out the rad and that is present in the whole system.
thank you so much for your answers. You're so lucky to still have a local hardwood store chris, and i love that you can buy something in there for 2p. Mine closed a year ago which was such a shame as i could get my little pup a treat in there for 5p.
Do you really think i should drain the whole system though youngmafbog? I was thinking about doing it, but it just seems such a waste of water just to be able to paint behind the radiator
If your issue is decorating behind the radiator then you may find that by slackening off the pipe connections a little, if the pipes are free enough to move up and down, you can lift the radiator off the wall bracket, swing the top forward and down, and lay it on the floor or boxes.