Perhaps that should be "in the field."
It's easy to find and choose campsites when you're planning. You can look at a map, search the internet – or ask here! But once on the move, it seems more difficult. The problem is not so much finding campsites as identifying ones that are acceptable to you and indeed that find you acceptable.
Story follows: Coming back from Long Itch a couple of weeks ago I had some trouble with this. I'd printed out a map of my route from the excellent cycle.travel website (as well as a gpx), so I looked at it and identified three potential sites, one before Tewkesbury, two just after. I went past the first one fairly early so went on to the second, which turned out to be simply a bare field with a notice and a couple of caravans which might or might not have been inhabited at that point. Well ok, I could use the "facilities" in the pub round the corner but at this point I really fancied a shower so I went on the next. This turned out to be a very large site with lots of caravans, and some static "mobile" homes, lined up neatly, but no sign of anyone in charge. No one had any idea when they would turn up, so I went in the nextdoor pub, where I was told it's a totally separate business and there's no way of predicting them. Well they must turn up in the morning, I suggested, maybe I could just pitch my tent somewhere and find them next day? Pub staff advised against this strongly enough that it was no exaggeration to call it warning off, seems the campsite owners don't like people arriving arriving without booking even when it's quiet, and I got the impression don't like tents or cyclists either. (This relates to the campsite not the pub. Also, I noticed that the site is for sale!) So I went back to the previous place, found the owner, and he very in a very apologetic and friendly manner said he was only licensed for caravans (presumably due to it being a bare field with no toilets etc) and couldn't risk losing his licence as it was his income.
By now it was beginning to get late, though still light. I looked at the map and found another campsite marked nearer Gloucester. If they also said "caravans only" or were closed or something, then I'd either camp under a hedge or ride into Gloucester and hope to get a train home (which would have been a shame as I was looking forward to riding home the next day!). Luckily, they were open and run by a friendly bloke with no dislike of tents or cyclists (and no attempt to charge caravan rates either). But I had no way of knowing that till I got there. Oh, and this one was not next to a pub!
(Tl;dr? Tried three campsites before could find somewhere to stay!)
Later, back at home, I looked at the OS map and only the first two were marked, not the one I ended up at. But mostly the problem seems to be identifying the nature of a site before you get there, rather than its existence. Booking everything in advance seems to run counter to the spirit of cycle camping for me (besides requiring too much planning for anything beyond the first day). Presumably an app is the answer; but which app? I know (cos it's linked from the cycle.travel site) of one called Archies, but what do people use? What do you do? Apps, maps, pot luck, what?