I asked the same question at my local council (Adur distrcit) the other day.
They were most helpful. They directed me to an area on their website that has the full details of the location of allotments in the area (together with all the rules and regulations!!!!)...
You can even put you name down on a list to get your own allotment via their website. I have a sneaky feeling the waiting lists are long, but the rewards of having the sanctuary of your own allotment certainly make-up for the pain of waiting...
As for pricing - allotments are measured in a fairly odd measurement called a rod - and it depends on how many rod's you have as to how much you are charged... currently 10.5 rods (more than enough for a year's worth of growing) is less than £50 per year... no wonder the waiting lists are very long!!!!