Fellow Denverite here! Welcome to WMT!
I am going to apologize in advance for such a long message, but I have a lot to share about what I've experienced with making wine in Denver.
I recently joined Colorado Wine Club (CWC), a club for winemakers, this past Spring right before COVID hit. You should email Mike Giannetto if you're interested in joining.
[email protected]. The group normally meets once a month, however we haven't met since February due to COVID.
As far as finding grapes/juice, I know the struggle. We are not in the best location, geographically speaking, for grapes/juices. Unfortunately, in my 3 years of making wine, I have not been able to locate a local source for juice pails during the off season.
In the Fall, there are a few options. As you mentioned, there is Castle Rock Homebrew Supply and Altitude Brewing & Supply. The other "in town" option is 5280 Produce. From what I understand, Altitude has the same supplier as 5280, but 5280 is much cheaper. I can't say the same for certain about Castle Rock. I myself have not bought from 5280, but I know a lot of the CWC members get their grapes from there in the Fall and they tell me the prices there are great. The last option is F. Colavita & Son. They DO have a "local" distributor, but they are located in Vail. I have recently been in contact with them in an effort to gather information. Needless to say, this Fall, I will be getting my grapes from either 5280 or F. Colavita & Son.
During the off season, we dont have many economical options aside from purchasing wine kits from brew shops like The Brew Hut. If you have your heart set on making wine from scratch, you'll have to be prepared to pay a premium. I recently bought 2 pails of frozen grape must from Brehm Vinyards on Oregon. The quality of grapes was great. However, with shipping, I had to pay ~$360 for 100 pounds of grapes. The other similar options I have been able to find for frozen grape must are Wine Grapes Direct and Musto Wine Grape Company. However, the pricing doesnt differ much. Shipping will still cost an arm and a leg. If you want grape must from Chile or South Africa in the Spring, Musto is the place.
Again, sorry for the novel, but hopefully the info is helpful.