I've had a bit of trouble at first with finding the symbiote samples in the game and after looking online for help I saw that there were more than just me who needed help on this matter. The problem was no solution... but I honestly think that I have figured out how to get the needed samples.

If you notice the hive's spawn certain types of symbiote. The tendrils inside of the giant cabbage patch will tell you which is being spawned.

Purple: Mini Venoms
White: Slashers
Magenta/Electric: Electro Symbiote
Red: Berserker's

Vultures are usually found on the roof but this mission is given to you way too early. After finding about two of the samples you have to advance into the game a little bit for the others to come.

It's just a matter of finding the nest the symbiote belongs to and destroying it.

I hope this helps, by the time i figured this out I was on my last sample so this has not been tested.