Stores that sell discontinued shades of OPI nail polish range from major online retailers such as Amazon.com to more obscure sources such as NailPolishDiva.com, NailsAve.com, MBeautyLounge.com and TheBeautyClutch.com. Amazon has a list of products compiled based on the reviews of users who searched for discontinued OPI nail polish.
Nail Polish Diva and Nails Ave both have large collections, though many may be out of stock. Nail Polish Diva also has a mailing list, so you can receive alerts when new colors become available. M Beauty Lounge and The Beauty Clutch have smaller collections. Prices for different colors usually vary by a few dollars on each website, as of 2015.