If the purchase is under $50 and you are certain your supervisor will approve reimbursement for it, just buy it and bring item and the receipt to the office. Clicking this link will take you to more detail on this process. (You could also ask for a cash advance to purchase the item).
If the purchase is $50 or more, you fill out a Purchase Requisition Form (PRF), get it approved, and give it to purchasing.
We’ve linked the form here: Purchase Requisition Form. Note, however, that your office may have hard copy, pre-numbered forms already. If so, you must use one of those. Check with the purchasing staff.
Instructions on how to fill it out are here: How to Fill Out a PRF
Information on approvals is here: Controls and Authority Levels