You can find smaller barrels for spirits, but I've never seen anything smaller than 25 gallons for wine. Most that I've seen are in excess of 50 gallons.
They are new barrels though. I think it would be fairly easy to pour a bottle of wine or whisky in one and let that sit to give it a different flavor profile.
Are you looking for barrels to age wine in or looking for barrels to age beer in? Your best bets are spirit barrels in 5 to 8 gallon sizes. These can be rinsed and a barrel solution added to remove the spirit flavor and aroma or you can put high ABV beers in them. In either case, these will be a lot cheaper than a new 5 gallon barrel. Balcone and Woodinville barrels are both available year round.
I have one of each. The Balcone came with a silicone stopper and a barrel stand and is 5 gallons. The Woodinville is 8 gallons and only came with a wooden stopper. They run about the same price (unless you watch for specials).
Small barrels are problematic. Evaporation and over-oaking are real issues as the ratio of beer volume to oak surface area is quite high. Small barrels can be good for flavoring a beer, but I wouldn't age in one.