Unicorn hairs would probably the easiest wand cores that we could obtain without running afoul of licensing laws, if only because there are some unicorns in the Forbidden Forest. Of course, the trick would be obtaining them. Hermione would be the only one of us who could approach them. Or perhaps Poppy has some, if they're used as potion ingredients?

We could ask Mundungus Fletcher, but frankly, I wouldn't be inclined to trust him if he pops up presenting something as a dragon heartstring or a phoenix feather. It'd be more likely to have come from a lizard or a parrot.

Emmeline reminds me that old time wandmakers sometimes used other cores, like kneazle whiskers. I'm not sure they would be as powerful or predictable, of course. Do we know anyone who has a kneazle?