On my daughters bike, the cover of the revo shifting lever is broken. The specifications of the bike say the shifting lever is SL-RS35, but when I look at images in the net, these look completely different. I uploaded images of the levers to http://imgur.com/a/6ESp1 Maybe someone knows which model this could be?
SL-RS35 is correct, there are newer versions however, there are many you can replace it with. Shimano uses a 2 to 1 pull ratio for shifters, Sram uses a 1 to 1 except for their entry level MX lines.
In layman's terms you will be fine with just about any twist shifter made for 3x7 speed, Shimano Tourney or Revo twist, Sunrace, or Sram MRX Comp
Compatibility should not be an issue with these entry level parts. There are also several on Amazon but I personally like to support my local bike shop.
Here are some compatible Sram MRX shifter for sale online http://www.ebay.ca/itm/Drehgriff-Set-SRAM-MRX-Grip-Shift-7-3-fach-/181693382982?hash=item2a4dc50146:g:DMMAAOSwGiRTqtpz
Located in Germany :)