There are eleven snakes to collect. Here is the earliest you can expect to find these creatures, and how to get them. Do note that the Tsuchinoko is a "mythical cryptid", thus not technically counting for this.
Snake A: King Cobra
Snake B: Taiwanese Cobra
Snake C: Thai Cobra
Snake D: Coral Snake
Snake E: Milk Snake
Snake F: Giant Tree Python
Snake G: Giant Anaconda
Snake H: Reticulated Python
Snake I: Snake Liquid
Snake J: Snake Solid
Snake K: Snake Solidus
Snake G, Snake H: Both of these can be immediately found after getting your gear in Dremuchij South. Look on the ground around you and either shoot it, cut it, or tranquilize it, making sure you pay attention to which pops up.
Operation Snake Eater:
Snake A, Snake F: In Dremuchij North, you'll come into this area differently from the first time. You'll see F hanging off of one of the branches on your way to the landing site. As for A, when you come to the fork that leads either north or east, take the north path and look around the ground for this snake.
Snake D, Snake E: In Chyornyj Prud, you'll come across these two on the southern bank before you even enter the water.
Snake B: The place you'll want to start watching out for this one is in Chyornya Peschera Cave Branch. Unfortunately, I'm unable to properly give you an exact location for this one other than that you will find it here in this part of the cave, before you end up going further in and encountering The Pain.
Snake C: After you've encountered The Pain, you'll start making your way out into the light once more in Chyornya Peschera Cave Entrance. This snake will be found on this map. For me, after I ventured out some, I found it lying in wait in the opening where there was a small cave to the left (with some claymores within).
Snake I, Snake J, Snake K: The last three snakes you'll require won't be found until the final battle in Rokovoj Bereg. Scattered throughout the arena are three fallen logs, and beside each of these will be one of the three snakes with the same name as those in the Les Enfant Terribles program (Solid, Liquid, and Solidus). You will see a tree python hanging off of a branch, but the snakes you're interested in will be hiding amongst the flowers on the ground. You might want to rely on your Thermal goggles, if you got them earlier, to help you see them between the flowers.