...it's hard to get ex-bourbon casks that small. Except from Tuthilltown. And BT, I suppose .
I can easily buy virgin toasted or charred barrels in 1, 2, 3, and 5 gallon sizes and so can anyone else. I obtain used casks by... using a virgin barrel and then refilling it. It's not any more difficult than that. I've done it and I think you've done it, too. Except for the expense, it's no more complicated than that. Small barrels are pricey.