Where do Palestinians find arms, when Israel keeps them under a strict blockade?

They smuggle them through Egypt and Jordan.  Maybe some other ways too.  They make border tunnels so they won't be seen.

When it comes to Gaza, it's important to know one thing: They have a border with Egypt along with Israel. Therefore, Israel cannot completely blockade the Gaza Strip alone, since Egypt will not be complicit with Israelis enforcing their own policies on their soil. That means that Egypt is blockading Gaza as well, for their own reasons.  

Of course, that makes it seem like Gaza shouldn't be able to get anything like arms through... but, Egypt's involvement is actually the reason. Egypt's standards for blockade are actually less stringent than Israelis, so the border along Gaza and Egypt is much more porous. They don't monitor it like the Israelis, so Hamas (the governing body/terrorist organization in charge of Gaza) can dig tunnels to smuggle ammunition in. It doesn't help that the border is out of its way and remote for Egypt, so it's harder to man and maintain for them compared to the Israelis who are right there. Also, it's worth mentioning that much of Gazan weaponry comes from Iranian sources, who actually have contacts in Egypt versus Israel who they're at war with. And the Egyptian populace may be more approving of violence (or "resistance") against Israelis versus Arab Israelis who have shown to be more nuanced and pacifistic in pro-Palestinian approaches.

As for the West Bank, it's kind of the same, albeit with Jordan instead of Egypt. However, it's also worth it to note that Israeli security forces have a better grasp so things like rockets weren't as much of a concern. Instead, it's smaller things that they smuggle in, like guns and suicide bombs in the older days. However, we also see attacks from these Palestinians using things that normally would go in, like culinary knives and cars.

As a Greek I prefer Palestinians over Israeli killers

They pick them up right off the ground. Lots of rocks and pebbles around.

AT THE ENDS OF THEIR SHOULDERS, I GUESS. All the Palestinians I saw in Palestine had arms like any other people.

They are a terrorist govt with ties to all the terrorists of the region.

Iran would be a good guess.