A metal shop or welding shop can produce one for you for around 30 bucks...depends on the shop...there is also another fun way to make a different kind of guidance system for the water coming out of the gutters.
A big thick antique (maybe) chain attached to the downspot coming from the gutter will 'hold' water to it as it flows into the barrel. Very pretty. If it is a major downpour the barrel itself will either catch the water and when full allow the water to splash out widely enough to not gouge a hole beneath the downspout.
Just an idea. Otherwise, I've had specially made fittings for woodstoves for instance. Look up Welders, Welding, Metal Smiths...tell them what you need and at least they will direct you to a shop that deals with making customized fittings. I was amazed how inexpensive this was!