My Raymarine Elememt 7 is a power-hungry fishfinder, that I use a 30-ah battery with from ampedoutdoors
https://ampedoutdoors.com/collections/lifepo4-battery
My friends with smaller fishfinders have been buying Miady lithium batteries off amazon.
https://www.amazon.com/LiFePO4-Batte...7-93deae8f9840
The beauty of amazon is competitive prices, and easy returns. My buddy has retured the Miady battery twice for replacements.