I got a Boss Re-20 Space Echo for Christmas this year, and i hve a question about it. In the manual, it says that if you use any adapter other than the Boss PSA adapter, it could "could cause the unit to malfunction, or cause damage to the unit." I was wondering if this was a baseless claim in order to get people to buy their gear, or if it could actually harm my pedal.
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Each unit usually works on 9v battery but the amperage may be quite different. Anything from 50mA to probably 500mA, so manufacturers have to protect themselves from mis-users by making the statement about other power supplies. That said, each device should have details of power input required and any matching supply should work just fine. Look at the voltage, Amperage and polarity (some devices are set for positive centre and some for negative centre), before you risk an expensive pedal on a cheap supply.
Boss also supply PSU's designed for several pedals which is a good idea ro reduce the number of mains devices. Then there are retailers such as Thomann in Germany who provide compatible supplies that carry their guarantee of conformity. It's worth browsing their online store if you are in Europe as the savings can be quite a bit.