While staying at Illyrio's manse in Pentos, Tyrion Lannister goes for a walk in the gardens and notices a poisonous variety of mushrooms growing in a cracked paving tile. He takes seven of them and stuffs them carefully in his boot. 
The seven mushrooms
Tyrion sits on a stone bench drinking a flagon strongwine. He drops the flagon when it is empty and it rolls across the yard. As he goes to fetch it he notices some mushrooms growing up from a cracked paving tile.
The mushrooms are pale white, with speckles and red-ribbed undersides dark as blood. He snaps one off and sniffs it and thinks to himself delicious…and deadly.
There are seven mushrooms. Tyrion wonders if the Seven are trying to tell him something. He picks them all, snatches a glove from down the washerwoman’s line and wraps them carefully, and stuffs them down his pocket. The effort makes him dizzy, so afterward he crawls back to the bench, curls up, and shuts his eyes. While it is not mentioned if Illyrio knows that Tyrion picked mushrooms, it is quite likely that he does.
While heading down the Rhoyne on the Shy Maid, Tyrion is in a bad mood and thinks to himself that he still has the mushrooms he had plucked from the ground of Illyrio's manse, and there are days when he is sorely tempted to slip them into Griff’s supper. The trouble is that Griff scarcely seems to eat.
After being kidnapped and being held captive by Ser Jorah in Volantis, Tyrion’s thoughts again go to his boot, to the mushrooms in the toe. His captor had not searched him quite as thoroughly as he might have. At the time Tyrion is under the (wrong) impression that Ser Jorah means to deliver him to his sister. Thinking of the mushrooms he says to himself,
Much later, after being sold on the auction block in Meereen, he and Penny (and Jorah) are on their way to their new master Yezzan zo Qaggaz. Tyrion has heard about Daenerys Targaryen, but he thinks to himself that he has no faith in royal rescues. He contemplates,
Later, when Yezzan zo Qaggaz's hated overseer Nurse contracts the pale mare, none of the other slaves will go near him, so Tyrion attends to him. Nurse succumbs to the disease; however, Tyrion is implied to have hastened his end and be the ultimate cause of Nurse's death via poisoning. When Nurse falls ill Tyrion serves him a nice hot dogtail soup “with slivers of mushroom in the broth,"  implying that the mushrooms used in the soup are the same poisonous ones that Tyrion took from Illyrio Mopatis’s mance. As he is feeding Nurse, Tyrion says to himself, drink it down Nursey, that shitwater squirting from your arse needs to be replaced.
The last word spoken by Nurse is "No," while Tyrion's last words to Nurse are,
As Tyrion only used a small amount of mushroom, he more than likely has plenty left. Presumably they are still in his boot when he signs up with the Second Sons.
A Dance With Dragons
Tyrion discovers poisonous mushrooms growing in Illyrio’s garden. He picks them and stores them in his boot.  He has them in his boot throughout his travels in Essos. In Meereen he serves his overseer Nurse dogtail soup with slivers of his mushrooms.