I followed Mr. Morgenthaler's advice and purchased my barrel from the guys at Tuthilltown Spirits in New York. I was able to get a 6-gallon barrel that had formerly held bourbon whiskey for ~$125 plus shipping. It is very important that you find a quality barrel for your process. For my aged Negroni I wanted a barrel that had been flame-charred and then had stored bourbon (or rye) whiskey for a number of years. And you want to get the barrel directly after its primary purpose as a whiskey storage vessel has been completed. You obviously don't want a barrel that has been drying out in someone's back forty for the past couple of years, and you certainly don't want the barrel your Uncle Cooter has been using as his card table for decades.
I spent a lot of time over the Summer months looking for other suppliers for barrels and could not find any other sources for whiskey-cured barrels. I even spoke to the folks at the Buffalo Trace distillery in Kentucky to see whay more distillers don't sell barrels to bars and other mixologists. It seems that many distillers sell their used whiskey barrels to lower-cost whiskey producers (Think Jim Beam and Canadian Club) to be recycled aging their whiskey products. As a result whiskey barrels are pretty expensive and a bit hard to find.
That said, the barrels are recyclable - so you may not need to buy more than one to be used on multiple projects. Now that my Negroni aging process is finished I just bought a couple gallons of cheap blended scotch whiskey to re-condition my barrel in preparation for the next project - a Trident cocktail. Other barrel-aged recipes call for wine-cured barrels (Mssr. Morgenthaler's aged Manhattan recipe in fact calls for a barrel cured with Madeira wine). Those may be easier to find if you live in a wine-producing area of the world.
When considering the amount of cocktail that you would like to produce, be sure to assess the cost of buying 10 or more bottles of spirits. Luckily, the spirits involved in a Negroni (gin, sweet vermouth & campari) are all reasonably priced at $15-30 per bottle. But even so you should expect to spend $200-300 in spirits, depending on the size of your batch.
The recipe below assumes a 3-gallon batch of aged Negroni. It resulted in just slightly less than 3 gallons of cocktail - the Angel's Share was approximately 1/2 a bottle of spirits. BE SURE TO KEEP YOUR SPENT SPIRITS BOTTLES. They will come in handy after the barrel aging process is complete and you need to store your cocktail in glass bottles until you are ready to serve.
Adapted from jefffreymorgenthaler.com
4 750ml bottles Campari
4 750ml bottles gin (I used Boodles, but Plymouth is also great)
3 1liter bottles Carpano D'Antica sweet vermouth
Combine all ingredients into a large bucket or mixing bowl (without ice). Stir and pour using a funnel into the barrel. Use a rubber mallet to hammer the bung (stopper) into place and store in a cool place for 4-6 weeks. Save your spirits bottles for storage when aging process is complete!
Beginning at about 4 weeks its okay to take a little sample of the cocktail to check on the progress of your concoction. I found that 6 weeks was just right in my case.
When aging process is finished pour the liquid out of barrel into a bucket or mixing bowl and then strain the liquid (to remove the charred wood chips) into the original spirits bottles with a funnel.
To serve cocktail, pour pre-mixed cocktail into a mixing glass with ice. stir vigorously to chill drink and strain into an up glass. Serve with orange peel if desired.