Unfortunately, tuna is not a healthy, complete diet for your cat, even without salt. Have you tried an actual cat food with tuna in it? Sounds like you have a "tuna junky" on your hands so switching her to tinned cat food may take quite a bit of patients 'cause she's probably addicted to the tuna. It's SO smelly and yummy!
But, I think, it's well worth the trouble.
There are several reasons tuna isn't good on a regular basis beside not being complete. It is very high in heavy metals such as mercury (which is toxic).The guideline for adult humans is any more than one can of tuna a week can cause toxic levels. Think what it can do to a little cat! Also the oils in canned tuna can leach vit E which can cause a condition called steatitis. It makes the nerve endings extremely sensitive and can cause a lot of pain. Then there is the whole issue of UTI etc..
A long way to say, salt or no salt, tuna probably isn't your best choice. But if you give it as an occassional treat, I would get tuna packed in water, not oil, without any salt. As for where to get it, I don't know where you are, but here in the states I know you can get it at Trader Joe's or Whole Foods for sure. I would think almost any large grocery store would carry it but I'm not sure.
Sorry, I didn't exactly answer your question but I hope it helps.