This is a basic item at any jewelry store. A 2.5mm lightweight half round band in size 6 should be $300-400. If you buy a cheap one, the finish may be haphazard. Platinum takes more steps to finish than gold, a quality band will reflect (pardon the pun) the care taken in finishing.
Also, eBay. I have a saved search that I get updates on for "tiffany lucida platinum," because I''m waiting for a ridiculously good price on the 2mm band. Stuff pops up all the time. I''ve seen a lot of good plain bands on eBay, both brand name and not.