Get a pawn with the forager vocation - you can get the tome by doing Kendrick quest at the slums or simply add a pawn with that specialization. This will add crafting materials location on the map - simply go to your map search for crystal icons press the corresponding key on the crystal icon (F on PC) and it will tell you what the source of that material is - example: Field (no monster) or a monster name like a grim ogre or a cyclops.

Now the trick is it will only show you materials that you need right now for equipped items for you or your pawn! so if you or your pawn are not wearing upgradable gear nothing will show up.

So what you need to do is equip an item that the next upgrade will be from a monster that you want to look for. For example if you want cyclops to appear go to Vernworth and buy a broadsword and upgrade it to level 2 because level 3 requires a misshapen eye then equip this sword yourself or on your main pawn and check the map. Another example - equip a weapon that requires a dragon scale as its next upgrade and it will show you all the drakes - molten fury(lvl2) , dragons bite(lvl2) and most other weapons with dragon in their name.


Additional notes:
if you don't know which weapon/armor to buy for a specific monster just look online on sites like fextralife for a material that drop from that monster and it will tell you which items use that material.

while in the map menu there is also a search key which will make finding a specific monster/material much easier.

Some monsters have some additional condition to appear - for example skeletons only appear at night.