some states let you buy them online some don't. Even in states that "don't" you can usually find some identical ones marketed as being veterinary insulin needles that everyone says are fine, but I haven't tried them personally. Lotta states let you buy them in person without a prescription, though the pharmacist might give you a hard time if its a chuddy area. As a fallback, reach out to harm reduction orgs in your area and see where they get theirs/if you can get some, because they will have some. A variety of sizes should work in a pinch so don't stress about getting the exact right gauge and length as long as its close, though you will probably have preferences.

I just bought them on Amazon…unless you mean size and stuff

You can buy needles online, some pharmacies are cool with it if you can't do online. I requisition/liberate mine from where I work. You should get a sharps container. Do not reuse or share needles is the official advice

Subq should be around 25 gauge and 5/8" for length, I use longer because for some godforsaken reason all our needle are an inch long and I'm comfortable using it. I've had to give insulin to some pretty cachetic people with diabetes, its possible to stick that needle pretty much anywhere if you can spot it right. Unless you have some injection training or experience, stick with 5/8" and the typical spots.