The Alivo is a Shimano brand. If you go to eBay and type in "Shimano Alivo 7 speed rear derailleur" not much shows up. I guess the lower priced bikes have all gone to Shimano Tourney as that has a lot of hits.
If you want to be certain your existing indexed shifters will work with the new RD, buy another Shimano RD. You need to observe whether the RD is direct attach (see here:
Shimano Tourney TX35 6 7 Speed Rear Derailleur Direct Attach | eBay) or uses a built in hangar (
Shimano Tourney TX35 6 7 Speed Rear Derailleur w Hanger | eBay) Look and see the length of the cage on the RD. They come in three kinds short, medium, and long. Which you need depends upon the range for the new rear cassette. If you simply replace the existing cassette with the same size, buy an RD of the same type as you have now. SRAM and Shimano cassettes are interchangeable on your existing freehub. It helps to have a chain whip to hold the cassette while you remove the outer ring that holds the cassette to the freehub. Of course you should use a Shimano cassette remover. Using the right tools makes the job a lot easier.