OSB IDE actually comes as part of the OSB installer. While installing Oracle Service Bus, one of the steps will be to provide Middleware Home & OEPE Home.
With this, it will install required plugins over OEPE to have the OSB IDE
Be aware about the versions of OEPE & OSB. Different versions of OEPE are used for different versions of OSB
You can get in Oracle website, or for quick reference
Here is the list
Hope this helps resolve the confusion over OSB IDE and OEPE.