Seems like there's alot of confusion dealing with NES and SNES power adapter (AC adapters, power supplies, whatever you call them) so here's (uh, almost) all you need to know of 'em. This is about european PAL models only. I'm not responsible if you break your console, yourself or anything else. Always doublecheck and if you're unsure of something, don't do it.
Q: So, what are the specs regarding power for my european PAL NES/SNES?
A: Officially, both take 9 volts AC. NES PSU is rated 9v or 9.8v 1.3A. Same applies to SNES.
Unofficially, you can plug in just about any 9V AC or DC adapter with any polarity into both NES and SNES and it will work (at least if the plug fits nicely in ;) and NES has been reported to work with as little as 410mA.
I personally use 9V DC 1A with my SNES without any problems, and actually, American SNES wich takes DC directly in (NEVER try to use those with AC adapter. NEVER!) is rated 10V DC 850mA which most likely is a bit overkill too and snes will work with less. In the innards of NES, that 9 volts is regulated to 5 volts, so it will work with up to 12 volts, with the side effect of regulator heating up more than usual, but it should not be an issue as the regulator is rated up to 18 volts. In theory, everything from 6V to 18V should work, but I'd stick with that 9V if possible. I have no information if this applies to SNES, and i advice you not to try anything than 9V or less, as SNES may have different power regulator (can anyone confirm or check it out perhaps?). There's also some talk of UK models having different specs but i cant confirm that and it sounds kinda silly to me, that there would be different PAL models. So, use your common sense.
Q: Should I buy overly priced (possibly used) official nintendo parts?
A: If you want to and can find one. I personally would recommend third party adapters, as both NES and SNES have somewhat rare sized plug. If you're buying international, check input voltage and wall plug so that it will work in your country. Other option is to buy those cheap adapters which have multiple plugs (most have right sized plug, some don't) and adjustable voltage. Or if you have broken adapter gathering dust, cut the plug from it and change it into any otherwise suitable adapter. You can even make your own 2-in-1 adapter that way, by getting proper plugs and wiring them to one power adapter.