Thanks for posting your question.
It seems that you have taken an unsecured loan from the lenders. In case the loan has been taken by cheque payment, then it must reflect in your account as well as the lenders' account, in which case you can draw up an agreement/promissory note regarding the amount lent. However, if the loan has been obtained in cash, then the lender will need to show the source of income before disclosing the entering into an agreement with you. Otherwise, the lender might get into a trouble with the income tax department.
As you have stated that the lender is also a person who has purchased some product from you, so you must have a proper documentation regarding the money taken otherwise your lender might show the money lent against the product purchased or as an advance of the product purchased which might hamper your business if you are not able to supply the same. Such situations may give rise to criminal liability as well in case one of the parties is not honest in their business transaction. Hence, you must get the above documentation/agreement/promissory notes drawn up in place as soon as possible. Since the document has to be signed by both the parties, you cannot create it one-sidedly. So you will need the consent of the lenders fir the same.
Even otherwise, I do not see a reason why the lender will not agree to it since it is his money which is at stake. The only reason that he might not agree could be that he has some dishonest intention regarding the transaction or that he does not want to disclose the income to the income tax department.
Do let me know in case you require any assistance regarding the documentation.