Merc suit is at the start of the game, stalker suit is in the Agroprom Underground, Barkeep then gives you another stalker suit, another stalker suit is located in Dark Valley, X18 has a monolith suit (there's actually two but one requires stash coordinates). You can also enable to have guns repaired (something that exists in the other two games).
If you're playing without mods, I recommend using this this will fix most bugs that are still in the game. Keep an AK/Viper till you get to the underground and get the fast firing AK, that alone should last long enough till you get your hands on some NATO weapons like the SGI-5K. Guns like the double barrel are pretty useless and should be replaced with an AK/Viper, once you have an SMG/Rifle you can dump pistols are they become very useless. I suggest you read up on the different weapons and suits also. You can use these maps to see where different things are located I suggest you read on the artifacts, using a jellyfish with nothing to counter the constant radation will just end up draining your HP.
