Concrete pavers of any sort are probably your best option. You can buy them in sizes up to at least 24x24 at the big box stores. Depending on your climate and frost line, you'll have to do differing amounts of prep to prevent them from heaving or shifting in the winter.
There are probably also plastic/rubber mat floors you can buy (i remember my parents' cheap plastic shed having them) and install fairly easily which would provide more moisture prevention than the pavers, but you may not be able to get the exact sizes needed easily.
Okay! I think I’ll try the plastic/rubber mat floors...it’s basically a temporary fix as I’ll never be able to install an actual floor in it. I’ll post photos tomorrow when there’s daylight and give a clearer picture, but basically what floor there ever was has rotted and most parts are just dirt. It looks like perhaps wooden pallets are at the bottom of whatever it was made of. Thank you for the input, it’s much appreciated!