I use Fish Emulsion Fertilizer a lot in my own garden. It’s organic, and most importantly WORKS. You typically use it for plants bearing fruit (such as tomatoes, watermelon, squash) and not leafy vegetables (spinach, Swiss Chard, etc). It makes the fruits a lot tastier from my experience. I typically buy it from my local farmer’s CO-OP, but it’s not an uncommon find elsewhere.

Here’s a helpful guide on using it: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/using-fish-emulsion.htm