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Companies enter into purchase / sale agreements all the time. ("We will buy from you X widgets per month at Y price, delivery such-and-such, notice for extra/fewer sales will be so-and-so, etc.") This is great, but it's not a strategic agreement.
A strategic agreement is when the two companies agree to work together at a much deeper level. They may agree multi-year plan in which the supplier will upgrade its equipment, or establish a joint research and development project to improve quality and reduce costs of the supplied parts, or even open a new factory especially to supply the purchaser. The businesses come close to depending on each other, although of course they should have a contingency plan in case the relationship (or the other company) fails.
There is no standard format because every such agreement is different. Any successful strategic agreement will involve close cooperation of production, legal, accounting and other departments. As with any legal document, the first step is to write down clearly what you want to happen, or what you have agreed. The lawyers then step in to formalise it.