Sometimes a good pair of snips or shears are all you need to get the job done. These lightweight Wiss snips are excellent for making straight cuts in silver and copper sheet. They are rated by Wiss to cut material up to "22-gauge cold rolled steel."
I have used them to make short, straight cuts with success on 22-, 24- and 26-gauge silver sheet. If you're strong, you'll also be able to cut 20- and even 18-gauge silver sheet, although the metal will curl a little and you'll need to flatten it out.
The wide handle loops give you good leverage and fit all hand sizes. We usually put one of the handles in a small vise fitted with non-marring vise pads – this gives you excellent cutting leverage (see second photo).