eBay Seller nopal_verde sells a variety of oak barrels in a range of sizes. Free engraving is available on some offerings. These ship from Mexico, but shipping charges are generally reasonable to the US. They also ship worldwide.
Check the seller’s eBay listings for current price, description and availability.
The smaller sizes could be dual purpose. You could mature a whiskey in it first. After that you have a used whiskey barrel you can use to age a barrel stout or other homebrew.
Also: Check AIH’s lineup of Oak Barrels – they have sporadic availability of new and used barrels
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Note from HBF: Depending on your location, you may need to obtain a license to distill alcohol