Speedstream 5100 Have Speedstream 5100 but no power supply and need to know wich one to buy

There should be a Rating on the bottom of the big label on the bottom of the router. From what I could read from the manual, it says: Rating, 12VDC 700mA and then a symbol like a half circle with a line around a dot with a line - one is +, and the other is -.
Whichever the dot was is the one you normally refer to (the one in the center of the circle). You can search for that adapter, but it's easier just to buy a universal adapter. Make sure it can go to 12 volts, and has a mA rating equal or greater than the rating on the router. Also make sure when you use it, that you have the center of the plug for the router on the same polarity as the dot with the line on the router - most universal adapters will allow you to change this polarity.

Hope this helps, please rate :)