Okay, I'm not sure if anyone is going to read my answer at this point, but here goes.
I don't think any of the poisons that have been suggested so far fulfill your requirements, largely due to the long time before death.
Botulinum is a particularly bad choice; although it has the dubious honor of having the lowest LD50 of any known substance, what it actually does is bring about a prolonged illness called Botulism, and that LD50 naturally is only relevant without significant medical intervention.
In practice, there is an antidote available, and with first world medical care (which there will be time to receive), there is actually a high chance of survival, even if a high dose is given. It's also fairly well-known and in hospitals, since people can get botulism even if they haven't been poisoned.
It may be fatal only weeks after the poisoning, if at all. One potential benefit is that it could be mistaken for a normal illness.
Polonium is certain death, in the long term. It also brings about a prolonged sickness which will eventually kill, though it is expected to take 20 days, making it unsatisfactory for your purposes. There is no antidote or treatment, and hospitals are unlikely to recognize or suspect Polonium at first because of how exotic it is.
(Though in light of the publicity it has received fairly recently, they might suspect it more readily than before)
How exotic? Well, to make some Polonium, you need to have a nuclear reactor. In fact, you need to have facilities specially dedicated to producing Polonium inside that nuclear reactor. There are only a few places on Earth capable of producing Polonium at all, and almost all are located in Russia.
It's also extremely easy to trace, to the extent you can identify specific people who have handled it. If you know what you're looking for, even minute trace amounts are detectable long after you've left the scene.
Ricin is the best candidate so far. It's almost perfect as a poison. It is relatively easy to produce (you could even prepare it in your home, though this is not recommended for obvious reasons), has no known antidote, and there is no reliable treatment whatsoever.
The only black mark against it is the time before death. The CDC says that:
Gastrointestinal effects generally occur within 6 hours of ricin
exposure. Effects on the liver, central nervous system (CNS), kidneys,
and adrenal glands typically occur 2 to 5 days after exposure and
reflect ricin's cytotoxic effect. Patients/victims may be asymptomatic
prior to the occurrence of these cytotoxic physical findings. Death
may occur between 3 and 5 days after the initial exposure to ricin.
This is a lot better than botulinum and polonium, but still a little off from the mark.
If you didn't have such a high onset of death requirement, this really would be the perfect poison for any job.
Although many people think VX is a "nerve gas", it isn't. It is a tasteless and odorless liquid almost exactly as a dense as water, and with the consistency of motor oil. It's more correctly referred to as a "nerve agent".
The "gas" form of VX actually refers to distribution as an aerosol, or from evaporation when an area is bombarded with liquid VX. It has a high boiling point.
Lethal Dose and Density Considerations
In mice, the intravenous LD50 is 7μg/kg, and based on human subject research, 20μg/kg should be a reliable lethal dose. For a 100 kg human, this is 2mg of VX, or 2μl in volume. According to this, mosquitos can carry between 0.001 to 0.01 ml of blood, which is slightly denser than water. As you can see, it's more than enough for this purpose.
Note that most sources for LD50 of VX for humans refer to contact LD50, which is significantly higher. It also takes a lot longer to take effect.
Time before death
In general, inhalation or intravenous VX can reach peak toxicological effects within minutes. After this point, the time before death depends on the dosage. With a sufficiently high dose, death can occur within 10 minutes to a few hours.
Although antidotes to VX exist, intravenous VX works too quickly for antidotes to be useful, unless they are immediately available. However, as VX poisoning has identical symptoms and treatment to other nerve agents, including more commonly available insecticides, it isn't unknown in hospitals. Tests for nerve agents are standard when poison is suspected.
VX is considered a weapon of mass destruction, and is recognized as the single most potent chemical warfare agent ever produced. It is not easy to produce, especially in pure form, and many of its precursors are also illegal and controlled, though none require a nuclear reactor and a team of physicists. However, considering the thousands of tons of this substance that have been produced to date by governments and subsequently destroyed, it isn't unlikely some of it has escaped into the black market.
- Molecules of Death, Waring, Stenton, Mitchell
- Chemical Warfare Agents, Romano, Lukey, Salem
- Chemical Warfare Agents, Marrs, Maynard, Sidell
- Death by Polonium-210