Google Forms is a popular tool that allows users to create order forms quickly and easily using pre-made templates or by designing their own forms. PDFs with fillable forms are also an option for creating order forms, which can be filled out and submitted electronically.
Legality: A contract is a legal document and a PO is a commercial document. To elaborate, a PO is an offer from the buyer to the supplier. If accepted by the supplier (within a reasonable timeframe) the PO becomes a contract. Logically, this PO will have zero value if it's not accepted by the supplier.