I will realy advise against buy in advance, or bulk buying in advance paint from anywhere other than the site/event/traders for the weekend.
Buy from elsewhere in advance and you may get a cheap looking price, but you have to add on postage to get it to you, or pick it up yourself - which means factoring in your petrol, and also carrying it from your car to the safe zone, camping etc. Then you have to store it making sure it is kept in the right conditions.
If there are problems with the paint on the day then thats your problem.
Buy from an event trader and you can still negotiate bulk savings if you are getting a lot - plus many traders will allow you to 'buy now / collect later' so you don't have to take all your paint for the weekend at once, take what you want and leave the rest in their storage and collect it fresh the next day.
Other advantages are that you don't have to guess what paint to buy in advance / can see how other people are doing with specific paints on the day.
You can either contact an event trader in advance if you have preferences/want a significant amount, or shop around when you arrive.