What is your country of residence?
If it's the UK, I would recommend Ric Stern.
Do you mean an additional PowerTap Pro?
In the US most good LBS's can get it for you. As a last resort, (at least for the last two years) USA cycling certified coaches could sell them. Some coaches with bigger programs have individual deals (in case the USA cycling deal is not renewed this year) with Graber/Cycleops where they can sell them as well.
If you just want an old grey CPU that downloads, I can sell you one.