Hi mgrlvr, thanks for posting the question.
Generally, you cannot deduct cost of purchases that you cannot support.
Below is a list of proper documents -
Quote
Documents reporting the cost of inventory include the following.
•Canceled checks.
•Cash register tape receipts.
•Credit card sales slips.
•Invoices.
These records will help you determine the value of your inventory at the end of the year.
page 12 of https://www.irs.gov/pub/irs-pdf/p583.pdf
For past cash basis purchases, you should re-construct a purchase ledger, listing items, dates, prices paid and location of purchases. However, they may be rejected in case of an IRS audit.
Hope the above helps.
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