Welcome to MountainBuzz. Please don't let the Buzzards scare you off.
This is a good place to hook up with other boaters at the same skill level and go boating together. However, your question is a bit confusing as you say you're looking for a "guide." To many of us, this has the connotation of a paid professional who will provide guiding services, is licensed in Colorado, may or may not provide professional instruction, and who will be liable to some extent for you on the river. There are many companies you can get for this and they will have half day trips. Unlicensed guides - like I would be if you paid me to take you out on the river - can face severe consequences if caught and especially if there's an accident.
gh suggested RMOC which is a great place to find guide services and may be able to rent you the gear you need for a good time out on the river.
Have a good time,
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse