Again, take a look at the label on your current power supply. Here's an example label:
With this particular example the output is 15 volts and 4 amps. So I'd want a power supply that puts out 15 volts and at least 4 amps. It could be more amps, but want 4 as a minimum. Next I look at plug polarity, and in this example it's negative on the outside and positive on the inside. The one thing missing from this label is the jack/plug size. If that doesn't match, you could always cut your old one off and splice it in. If you do splice in the old jack, just make sure you match polarity of the plug (using a multimeter).