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Vendors that don't take PO's
If less than $500, buy with a credit card, then put in the receipt, credit card transaction record from your account, and packing slip if shipped. Alternatively, you can cut a check and send that to the company, a cumbersome process. Finally, if someone has a P-card, they can use that. But be warned, you need to submit the receipt and packing slip (or other confirmation of receipt) within a very short time frame (3 days?), and these cards are prone to leading to all sorts of problems.
newegg usually has the latest products at excellent prices, but since 2012-2013 has been cluttered with lots of non-computing, consumer items. Good return policies.