I'd guess that most people use a commercial drink that is based on maltodextrin, e.g. Hammer's Sustained Energy or Perpetuem. Both work well for me.

One person I know buys it in bulk from J.M. Swank. A 50 lb bag costs $40 plus $20 shipping. You need to decide what DE (dextrose equivalent) you want. The higher the sweeter. Check this out:

http://www.varied.com/food/maldescr.html

I recently got done experimenting with what I think (it was given to me) was M500 on the product code. That has a DE of 9-12. When I order a bag for myself I will probably go with the M180, with a slightly higher DE. This is break down of what DE is used for the different applications:

http://www.varied.com/food/foodap.html

The form is a powder that easily disolves in water. Some of the endurance riders I know either use it straight, or mix it with a protein powder. The stuff I was given was the malto+protein mix.

Obviously this is stuff you want to experiment with before heading out for a long ride.