O looked into Ag flying years back when i first started to fly, as with an agricultural bacjground i wanted to put the 2 together. didn't prove much of a starter as my flying career went elsewhere. I was in aus/NZ at the time. Ag flying over there is mainly alomg the lines of topdressing in NZ, and rice sowing/spraying in Vic/NSW. General way in is as a loader driver for a few years, then as the vacancies come up some of the easier work until you get the experience. At least from the NZ view the 'golden age' has been over for a few years now and it's the well established owner operators doing the work. NZ also has a strong general link with flying and farming, so it's the boys on the stations that get the work.
Aus is a pretty similar scene with the rice work. The old hands have a world season of several months in Africa, followed by a few months in Aus and then Europe. Not much (none?) in the Uk due to the Food & Environment Protection Act (1988). One of the points that sticks in my mind is that you need to inform the local piloce and the local
bee-keepers liaision officer 24 hours before you inted to operate!!
Try <a href="http://www.agair-update.com" target="_blank">www.agair-update.com</a>
As for banner towing, there doesn't seem to be much about, if looking out the window is anything to go by. I used to work at Exeter Flying Club, and they had a whole set of banner equipment stored away that hadn't been used for a number of years. Might still be there. Make them an offer and set up for yourself!!