You can normally buy green beans from a local roaster. Look in your yellow pages or equivalent. There are also numerous companies selling green coffee on the web. Some roasters find it a novelty and will be helpful. Some will be offended at the idea that you think that you can produce good coffee in your home. Once you find a roaster that is willing to sell you green coffee beans expect to pay around 15% less than for their equivalent roasted beans (which is about the amount of weight loss in roasting). If you go through a provider that specialized in green coffee you may save substantially more.
Some coffee beans are harder to roast than others but with a little practice you will be getting results that will thrill you and your friends in no time. The freshness alone will over shadow most imperfections in roast process when comparing home roast to a coffee purchased at the grocery store.
When you get the green beans, store them in cloth bags in a cool, dry and dark place allowing air to circulate. Leaving them in the plastic wrapping for months can make them sweat and go moldy.
The vast bulk of the world’s beans are of mediocre quality. Green coffee importers will generally either specialize in “commodity” coffees that cost little and will taste okay, or specialty coffee, typically purchased in micro-lots. The difference in price between the top quality green and merely average is small, unless they begin getting into the highly rated and reviewed coffees and direct trade.
Your local roaster may have different priorities from you in choosing greens and may be less concerned with quality than price. How do you tell? A list of “standard” coffee beans (eg. Colombian, Kenyan, Sumatra Mandhelling) is a bad sign; more specific descriptions (Colombian Huila San Augustin, Kenya AA Kathangariri Estate) are much more encouraging. This in not to say that you can’t get bad coffee with an impressive name so find a green coffee merchant you trust.
Finally, don’t be afraid to invest in a proper coffee grinder, which can be one of the best things to spend your money on to improve your coffee experience. Coffee is best as a fresh product and roasting it at home is absolutely one way to improve the quality, but you’ll want to leave it as whole bean as long as possible and grind it just before brewing. This ensures that all the time and effort you spent in making a quality coffee doesn’t get wasted by leaving it to go stale.