The AddIDE board is a very old product, so I doubt very much that it is still available new from any of our Amiga Online Retailers, but you can probably find one used, or use one of the many other products that provide an IDE controller for different Amiga models.
Kipper2K is producing something called IDE68k that I think is similar to AddIDE(with RAM). I had AddIDE back in the day, and it was nice but would have been better if it came with 1meg of additional RAM.
http://www.kipper2k.com/I don't know the details because I don't have any OCS amigas except for original Minimig.
