From personal experience I can state, it isnt easy to locate a PS306. My first suggestion is replace it with anything.
IF you have a copy of the original code that is now on the PS306...all of the code, I think there are 3 files involved, then you can purchase a newer model from Klockner-Moeller (look at
http://www.moeller.net). I have had bad luck with the 306 and 316 but great support from Klockner-Moeller. I cant say anything about their new line but I am sure its better than the old stuff. Anyway, if you have the original file Klockner-Moeller will assist in converting it to work on their newer models, they may even do the conversion.
If you need to talk to a Klockner-Moeller rep then try the guy in Birmingham, Alabama, he is sharp and does training on their stuff. I know the last name is Lewis but cant think of first name. Give them a call, its may be the easiest and cheapest option.
If you dont have the original code (or cant get), in print or on a computer etc, then buy anything you want and start from scratch. I was willing to pay to retrieve the code and use in a new system but they will not or can not do it.
BTW:
http://www.plccenter.com sells used PLC's, I didnt see a PS306 listed but they do have a PS316 but that is a modular/rack mount type and not sure if they have the necessary I/O, worth checking into though. The program from a 306 would load direct into a P316 if you have the file (in this case just one file is needed) and sucosoft30.
You dont happen to be dealing with a WTI do you?