The best replacement for purchase order is invoice.

The best alternatives to purchase order are rated as following:
★★★★★  invoice
★★★★★  contract
★★★  sales order
★★  requisition
★★  purchase requisition
★  delivery order
★  subcontract
★  work order
★  change order
★  voucher
★  purchase request
  check request
  blanket order
  non purchase order
  sales invoice
  quote
  invoice first
  purchase agreement
  purchase invoice
  customer order
  po

Your alternative for purchase order is not listed? Suggest!
Speaking about purchase order, one probably shouldn't forget about goods receipt.

Other search terms related to purchase order and their ratings are:
★★★★★ goods receipt
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Google Trends

Scale is based on the average worldwide traffic of purchase order in all years. Click on graph for more.
purchase order
invoice
contract