Where did Jesse get the ricin from?

Question

In season 3 of Breaking Bad, towards the end of the season, Jesse learns that Combo was killed by two guys that are working for Gus. He tries to convince Walt to give/make the ricin so that he would put it in the hamburgers that Wendy would then deliver to their car.

But Walt never agrees to such plot, and refuses to cooperate. Then Jesse says:

I will do it with, or without you.

And then we see Walt and Saul trying to figure out a way to keep Jessie from committing his plan. While at the same time, Jesse is confirming the plan with Wendy. The next day, we see Jesse and Wendy waiting for the two guys to show up. They have hambrugers with them. It is inferred that these hamburgers will lead to the death of the two guys.

Where did Jesse obtain the poison from? Did he make it himself? Is it a different kind of poison? (highly doubted since he confirms to Wendy that it is untraceable, thus it must be risen) Did Walt give it to him (even reluctantly)? but then, why give him the poison and then plan to have him arrested?

It does not make sense...

Answer 1

Jesse got the poison online. He says so to Wendy himself in Half Measures:

I found this stuff on the internet. Takes days to kick in. Just keep quiet and this won't ever come back to you.

Since he has no reason to lie to her, really, and the type of poison is not elaborated on, we can surmise that this is a slow-acting poison that might be untraceable. We don't really know if he bought ricin or not, or even if the poison would've worked, since the plan didn't come through.

While it seems dangerous to buy ricin online (for fear of sting operations), there are recipes available on the web, so Jesse might've meant he found those there and made it himself. Jesse has already been in the meth business, and around Walt, long enough to know a thing or two about chemistry (and again, even if he botched it, we'll never know anyway, but the risk of it working was enough that he had to be stopped).

Answer 2

According to the wiki, Walt prepares the Ricin twice.

This answer explains that the Ricin wasn't used by Jesse as it's still in play.

There is speculation that the ricin was bought on the internet - or that it wasn't Ricin at all. Brock later gets poisoned by Lily of the Valley which the hospital cannot detect.

Conclusion is that Jesse didn't use Ricin to poison the hamburgers, but Lily of the Valley, which is found in Walt's house. (Which is shown in the ending scene of Face Off) - or he used an unknown poison.

At the very end of the episode, the camera zooms in on a Lily of the Valley in Walt's back yard, beside his pool

Edit

As pointed out in the comments, it wasn't suggested that Jesse knew anything about Lily of the Valley in Walts garden, but as this article states (Breaking Bad is filmed in Albuquerque);

So it wouldn't be shocking to find it in a garden outside of its native range. Like, say, Albuquerque.