Potato salad, corn on the cob, macaroni and cheese, green beans, brown beans, corn bread, rolls, garden salad
For desert, strawberry shortcake, pound cake with ice cream, fruit salad (I have a yummy recipe-hit me for details if ya like)
Allrecipes.com can give you ways to cook all of these, (I got a great Mac and cheese recipe for crock pit from there) and you can adjust all of them to feed forty people, then you can transfer the recipes to a shopping list, it'll even add all the duplicate ingredients together so you know the total amount you'll need.
Those are all easily prepared and relatively inexpensive.
There are all different kinds of sides that you can have with your pig. Most people like to have a vegetable. You can then choose some rolls, potato salad, coleslaw, a salad or whatever else you like with it. As far as portions go for about 40 people, it would depend on the amount of side dishes that you have.