I found two crab spiders, one on quest in mines and second armored on the east from Novigrad.
No one from them droped crab spiders venom, so where can i found crab spiders to drop venom from them?

I've played the game all the way through and never encountered any kind of spider in the game at all. I did see spider webs in a few places but no spiders to cast them, although at least one of the types of endrega insectoids can cast a web. I did encounter crabs on a beach but those are crabs not spiders and they're just graphic filler to make the environment look more immersive and while you can kill them with igni, they are insignificant and don't drop loot or have any other interaction.

So basically, there are no crab spiders in the game to my knowledge unless I totally missed some quest or something (totally possible). Having said that, is it possible you encountered one of the various forms of endrega insectoids and are just calling them crab spiders as they looked like that to you, or are there actually creatures in the game called this that I missed, and if so can you describe or post a screenshot as to where they are exactly? I'm curious. :)

Is the game telling you that you need crab spider venom, or where does that come from?

I hate the fact that there is a possibility I might have overlooked something in the game. :)

Edit - update: I just did some googling and is it possible you're referring to the Arachas? I encounterd those but I've never seen the game use the text "crab spider" anywhere while playing, however a Witcher wiki I found says that Arachas are a form of crab spider. It's possible that that was what I was talking about above when I mentioned endrega with web, I think I got two insectoids mixed up as they usually are encountered together.

It's arachas in English version. I spotted a few where there were endregas. Can't remember exactly where, but probably forests of Velen. If there are endregas chances are that arachas will be in same place too.

Ah, ok. I wandered around the game doing other stuff after my last catch up in the forums here and I managed to run into an Arachas in Velen and I got the venom from it, it shows up in the inventory as "Arachas Venom" for me. Unfortunately I didn't take note of my exact location at the time or I could have shared the info. If I stumble upon another though I'll try to remember. Might have better luck fast travel surfing to herbalists though. Interestingly enough I discovered a new herbalist North East of Oxenfurt that I somehow managed to not notice in my game playthrough too. :)

Krabopająki :D You can find them on Skellige, on the shores. Na brzegach Skellige szukaj. Dziadostwo jest mocne i groźne, więc uważaj.

Crap People! Crab People!

You can find them just not far south of Crookback bog on dry land and west of the guarded treasure. If you encounter the fiend you have gone to far south go back towards the bog.

On the main Skellige Island you can find some venomous ones. They are not to far to west of a line drawn between the 2 druid camp locations. I also found them in the northwest of the big island close to where I was hunting witcher gear. On Skellige you will also find drones which contain the venom you need.

There is a single weak one that respawns from time to time North of Toderas/South of Marauder's Bridge.

Not very tough one (only level 11, not "venomous", not "armoured"), so can be taken down early on when you need the potion ingredients.

Some of the others are much nastier.

There is a level 21? Armoured Arachas north of Novigrad in the woods above the monster den. This one is a bit tougher, and has nearly twice as much armour as the weak one.

There are some level 31 Venomous Arachsae in Skellige, which are distinctly unpleasant (especially when found in confined spaces... not looking forward to that fight on DM).

OMG! Stuff is now in Novigrad! @_@ Brace yourselves!