I’m not sympathetic to Hamas, I am merely addressing that it is not so simplistic as Israel chooses to spend money on defending civilians but Hamas doesn’t. The devil is in the details.
Israel is a legitimate nation state receiving $3.3 billion per year in military aid plus free technology sharing and cooperation between the U.S. Military and IDF vs Hamas, a proscribed terrorist organisation that must finance itself through criminal and illegal activity and then is very restricted on what they can realistically smuggle through a land, air and sea blockade while under constant satellite and UAV surveillance. As you said, even if they chose to buy an Iron Dome/PAC-3/NASRAMS/ etc type surface to air defense battery, who would sell it to them? Especially since you know they’d use it to defend Gaza by shooting down IDF aircraft, piloted or unmanned. The answer is no one. No one would risk the wrath of the US if they sold something that would make the military power more symmetric between Israel and domestic terrorists. It would also be morally wrong to do so imho.
On the matter of bomb shelters, I am sympathetic to the plight of Palestinian civilians. Gaza is located on a sea shore of mostly sand and no bedrock. In this landscape, you can build narrow, concrete reinforced tunnels. You can do a bit of Swiss cheesing, but there isn’t enough land to build large, deep, reinforced underground bomb shelters (bunkers) for civilians without collapsing all of Gaza Strip on itself. It’s just basic engineering. So again, it is not so simple as Hamas has “chosen” not to build bomb shelters. They couldn’t even if they wanted to. And the fact they cannot, should not be used as ill conceived proof that they care less about Palestinian civilians than the IDF do.
As for why civilians cannot crowd into the smuggler tunnels used by Hamas, there isn’t room which is basic physics. Secondly, Hamas should not do this as it would violate international law as by mixing civilian objects (bomb shelters) with military targets (terrorist tunnel network), often referred to as the war crime of using human shields. It’s quite off to be criticising Hamas for following international law by not using human shields in their tunnels, not that they don’t commit war crimes, but let’s not egg them on shall we?