You can and should purchase uniforms before you are in DEERS. Take your signed commissioning certificate to any military uniform shop (incl. NMCRS thrift stores, if that's your thing), and tell them you're a newly commissioned naval officer. Recommend buying 1x set khakis and 1x set NWU Type IIIs.
(Unfortunately, until you are in DEERS, you cannot be verified for shopping the NEX's online uniform store. But this site is also a good - if unorthodox - way to check whether you are in DEERS yet or not.)