You can write it yourself, if you know what you want it to say.
If you download a form for 20 bucks, that may help give you ideas on what sorts of things you want to discuss in the contract. But downloading and signing one without reading it and deciding if you want those things, seems a lot less logical then just agreeing with the other side on what you can think of and writing it in plain english.
e.g. A contract to sell arba minim, written by a layperson:
Contract to sell arba minim
Henche will sell Dan's arba minim. Dan will supply arba minim for sale, and henche will sell and will receive a 10% commission on all sales. If any are left, henche may return them for full credit. Henche has full discretion on setting price, so long as it is at least 1% above Dan's price. If henche or Dan wish to cancel, they may on 1 day's notice. Henche may not sell for anyone else this year, even if he cancels this deal. However, if Dan cancels, then henche may sell for someone else.
See, the contract above is probably binding in any state. It has bilateral consideration (henche is giving up right to sell for anyone else, Dan is agreeing to let henche sell for at least 1 day.). And anyone could have written it, as long as they thought of what they want the deal to be.
Disagree. This sort of contract will state what the basic agreement is, but often will omit necessary terms, and contingencies in the event things don't go as planned.
1. How much does he have to supply?
2. What quality?
3. What if the goods are less than the agreed upon quality? What if they aren't kosher at all?
4. What if the supply isn't enough? Can henche buy from someone else?
5. By when must the goods be delivered? What if they aren't delivered on time?
6. What if they are damaged in shipping? Who bears the risk?
7. What are henche's remedies if Dan doesn't deliver, or delivers fewer than agreed, or lesser quality?
8. What are Dan's remedies if Henche supplements from another supplier?
9. Is payment due on order, delivery, or sale to consumer?
10. How does Henche collect his commission?
11. What sort of accounting must Henche supply to Dan? How frequent? How detailed?
12. How much effort must Henche exert to sell the goods? If he doesn't market them in good faith, can he still get a refund for the unsold quantities?
13. If there's a dispute, can one party bring the other to court, or must they go to beis din?
14. Is the contract governed by civil law or Jewish law?
15. If there are any left erev yt, can Henche sell for less than cost? Is he required to? Does he get any commission from those sales?
I can go on...