Searching model number of Shimano shifting lever
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.

http://www.amazon.com/s?ie=UTF8&amp;page=1&amp;rh=i%3Aaps%2Ck%3AShimano%20Revoshift

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 :)