It seems that the two main shop quest items people can't find are 2 Shrunken heads for armour, and 1 aconite for fast poison.
This topic is to give locations for those two items or other shop quest items. The only location I know of for shrunken heads is in the Romulos Mission: The sixth day. In the last area on the right side of the large hall
Thereís a lot going around about Aconite. I can confirm that you only get it from Borgia Couriers (not even pickpockets). I looked up topics and tried to figure out how to get some. I have played this game for quite a while, and enjoy wandering around. I tackled lots of Borgia Couriers, at least a dozen, and not gotten Aconite. Obviously, if you find them when youíre not in a mission (particularly one that doesnít require you to maintain absolute stealth) and if youíre out in the open, theyíre easy marks. Otherwise, not so simple. Still, Iíd chased down every one I could and no Aconite.
So I searched and found topics, all dealing with Sequence 4, Memory 6. So, I tried it. Being a statistics sort of person, and also trying to be maximally helpful/honest, hereís what I found:
First, the credits. What I found seems to be ultimately based upon something that Ashfire posted on a different, but I think related, website (Gamespot). Looks like, later on in that same thread, tjacks found a potentially better way of doing it, but I havenít checked it. I actually just found that all and the links trying to give credit where credit is due. So first, credits to Ashfire, for posting the first attempt to help everyone out trying to find Aconite. Second, credit to those who posted it, followed it, and tried to improve upon it, including Lenskop, Mongey, UnKoWnG4MeR, Lioyd_the_hero, and SoPoF.
This does seem to work to generate a Borgia Courier. Like the original poster, Iím thinking that it always generates one nearby; you just may not see him before he leaves the area. To get to the area, I advise that you: 1) jump onto the wall by the Courtesans; 2) jump onto the roof beyond; 3) run to the Leap of Faith point and leap; 4) get out of the hay; and 5) head down the street (away from the hooligans). Iíve seen Couriers spawn/appear all along that road and/or the courtyard immediately to your left (with the courtesans). Walk (donít bother running or sprinting yet) down the road and you should see a Courier. Results below (somewhat in hopes that you donít lose faith and/or think that youíre having odd results; Iíd be happy to hear of your experiences).