In which store are you looking for it? Willy sells it down at the beach in his cabin

Crab pots give fishing xp, but you need some to unlock them.

Buying fish soup from Willy gives you a one level boost for long enough to gain enough XP to get to level 2 on your own.

The training rod is definitely not a new addition. I used it in my first game a couple of years ago because I found fishing too hard. There was a dialogue where Willy asked me how I was finding the fishing, and one of my options to reply was "it's too hard". Then he told me to buy the training rod. I don't know if it was available before that; I never looked. But try talking to Willy when you see him and maybe it will come up?

The lake outside the mine is a good place to start. You get carp there, and sometimes they never leave the position your bar starts in, so you don't even have to do anything to catch them - free XP!

Try going after carps. They’re available all year and even found in places like the sewers (if you’ve unlocked that already). https://stardewvalleywiki.com/Carp