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Crafting and Harvesting Poison (page 258 of the DMG) says that a creature must be dead or incapacitated before you can make a DC 20 nature or poison kit check to get a single dose of poison.
However, it seems to me that this system is for collecting the venom of random encountered creatures. If I have a snake familiar, could I not just command the familiar to bite a jar with a thin leather membrane to collect the venom? Would there still be a need to incapacitate the snake or make a check?
RAW, your familiar must be incapacitated:
The creature must be incapacitated or dead.
Since your familiar can’t die, and you can’t reduce its hit points to 0, we’ve got to find another solution.
Fortunately, there is an easy one: your familiar still needs to sleep.
From the sleeping rules listed in Xanathar’s Guide to Everything, we see:
While a creature sleeps, it is subjected to the unconscious condition.
And from the Player’s Handbook,
An unconscious creature is incapacitated.
So wait until your little buddy is asleep, and proceed as described in the Dungeon Master’s Guide.
Alternatively, a generous DM could just rule that since you can communicate telepathically with your familiar and they are friendly to you, you can just do it. But there is a way to do it RAW with little trouble.1
1You should still discuss this with your DM. The question remains of what happens to harvested poison when your familiar disappears or changes form. A similar problem is addressed in this Q&A concerning harvesting materials from a wild shaped Druid. Differing answers on that question were well received, and it seems there is no RAW answer, so this is probably best left up to your DM.
In real life (yes, I know :-) venom is harvested from live creatures (snakes, spiders, etc) with no lasting damage to the creature. Sometimes it is a fairly simple operation, for example jungle dwellers rubbing arrow points on the back of poisonous frogs.
If the creature is unfriendly then you have to incapacitate it somehow (which might require killing it, if the creature is big and nasty). If it is friendly, this is not required.
An ability check, probably DEX (Poisoners kit) or INT (Poisoners kit), will still be required to see if you accidentally get envenomed during the process.
What happens to the venom when the familiar disappears into its pocket dimension?
The same question applies to harvesting resources (venom, hair, feathers, etc) from a shapechanged druid - answered here.
