Slime Condensate is a surprisingly useful resource in Genshin Impact. You will need it to level up and ascend characters like Lisa, Venti, and Xiangling, and weapons like the Amos Bow and Skyward Blade.
Slime Condensate can drop from Slimes, and is a set type of resource regardless of the elemental type of the Slime it drops from. Slimes are also pretty common enemies, popping up regularly all over the game. If you do find yourself in desperate need of it, and wants somewhere to farm, then there are two good locations, one in Mondstadt, and one in Liyue.
In Mondstadt you want to check the area to the southwest of the Thousand Winds Temple. Slimes are regular spawns here, and will often pop into existance as you go to pick a flower, or open a chest. You can double up your farming in this area and bring a Cryo user to be able to pink the plentiful cryo plants as well.
In Liyue, you can find lots of Slimes on the penninsula and beaches to the west of the Guyun Stone Forest.
Remember, Slimes will just drop Slime Condensate, but you can use this at an Alchemy bench to upgrade to rarer materials like Slime Secretions for higher-level Ascensions. It will take three Slime Condensate and 25 Mora to create a single Slime Secretion. You can then use three Slime Secretions to create one Slime Concentrate. You can find Alchemy benches in Mondstadt and Liyeu harbor.