Don't know about shops in your neck of the woods, but mail order is a good idea - www.towsure.com.
There is already a thread on the question of what to take, but I don't find the search facility very useful, so here is my contribution again. Things that will stay in the van -
Battery
Spare wheel + caravan jack
Gas bottle (s)
25m mains cable
Stabiliser
Towing mirrors
Aquaroll or other fresh water container.
Waste water container.
Door step
Security device(s)
Picnic table and chairs
Fire bucket
First aid kit
Sink drainer - see excellent article in PC mag or you could end up buying a pup!
Things you can 'borrow' from home but having them in the van is better -
Crockery & glasses (preferably non-breakable)
Cutlery
Pots and pans
Low wattage kettle
Sleeping bags or duvets
Other essentials listed by other contributors -
Tool kit with basics like different screwdrivers, stanley knife. A cyclists all-in-one toolkit is very versatile.
Membership of at least one of the caravan clubs.