The EOS Rebel T7 is able to use any Canon EF and EF-S lens. You may click on the following link to view our currently offered telephoto lenses where some of them have a focal length higher than 300mm:
It's not that I want to discourage you from a lens purchase or upgrade but it is generally best practice and advice to keep all your gear essentially in the same quality level. I.E., people usually don't put a multi thousand dollar lens on a Rebel T7. Perhaps a better choice is one of the very good 150-600mm super zooms form Tamron or Sigma. I'll bet both you and your bank account will be thrilled.
Sigma make lenses for many camera makes and models - they are what is called a 3rd Party Supplier. You would just need to look for, or order a lens with an EF mount - it will then work on your T7.
I enclose a link to Sigma's current telephoto lens line-up. HERE If you find a model you like you can check reviews for it on DPReview.com and if you shop around you may well find a cheaper deal - especially if you carefully check out the 2nd-hand market.
"You would just need to look for, or order a lens with an EF mount - it will then work on your T7."
There is a small caveat to that statement since the suggestion of a used lens has been made. You should only be considering the more current used models of "EF" or EF-S" lenses made by Sigma or Tamron. There can be some compatibility issues with the much older used models. Of course any current new or used model will be just fine. Thousands upon thousands of them are in use today. Neither Sigma or Tamron calls their lense "EF" or EF-S" so you need to read the description carefully to make sure you are buying a Canon mount lens.
SIgma and Tamron are not the only makers of lenses but they are the only two companies that I can wholeheartedly recommend. Tokina is another maker but their lenses are so spotty as to which model is a good choice, stay away from them for now. There are some totally manual lens out there too. But again stay away from them also.
It is best to stay Canon but I realize sometimes cost is a big factor. Most of us just can't spend the money big teles from Canon cost.