There isn't really an agreed upon definition of a "boutique" wine or winery. Some salespeople say that if a winery makes less than 2,000 cases a year, it's considered a boutique winery. The California wine Institute [www.wineinstitute.org] doesn't really use the term, but breaks down the 900 (approximately) wineries in California as follows.
By logical extension, those making under 5,000 cases a year would considered boutique wineries. Some of the bigger wineries produce a very limited number of cases of a particular wine, which could be considered a boutique wine. Webster's defines "boutique" as a small shop or specialty department within a larger store.
My definition of a boutique wine (not necessarily winery) is a wine that is made in limited quantity, and expresses a particular passion of the winemaker.
There are a number of examples of that.
Louis Martini produces about 150,000 cases of wine per year, but only 800 cases of Folle Blanche, which is a very rare grape outside of the Cognac region of France, where it is used for blending. In California it's almost non existent, except for Louis Martini. I might add this is a shame, because the notes on this wine are "Clean and crisp with flavors of lemon blossoms, honeysuckle and grapefruit, yielding a creamy mid-palate with a slightly sandy finish." And it is all those things! It's a wonderful summer wine, that retails for about $17, and is well worth it! If you're interested call Louis Martini at 800-321-WINE. They can tell you where to get it.
Another great boutique wine find is Tempranillo by Turnbull Winery. Turnbull produces from 20,000 to 25,000 cases of wine a year, but only 40-50 cases of Tempranillo. You can get this wine at the winery or through their wine club, or if they have any left, you can order it. It retails for about $35 dollars a bottle, and is worth every penny of it. You can call the winery at 800-887-6285.
A couple of other examples are Floodgate, which produces 2,000 cases a year of Pinot Noir and GewŸrztraminer combined ($19-$30), and nothing else. Even though they have their own label, they're part of Belvedere. Another Belvedere winery with its own label is Bradford Mountain, producing about 4,000 cases of Cabernet Sauvignon and Zinfandel (approximately $40). These boutique wines are rated very highly.
From Australia a couple of boutique wines you should try are Hewitson's Mourvedre Old Garden ($35) and Joseph's Cabernet/Merlot moda Amarone ($35). Other wines by either company are always good bets too.
From Italy you can try, Sanct Valentin's Sauvignon ($45) & GewŸrztraminer ($45), from the Alto Adige region, and from Tuscany look for Machiavelli's Ser Niccolo 100% Cabernet Sauvignon ($75).
There are boutique wines from all over the world. What you are looking for in a boutique wine is not just a small winery, because there are many small wineries that are not producing good wine or interesting wines, but due to their size they have to charge more for a wine you can get for less from a bigger winery. What you are looking for is the reason why a passionate winemaker makes a slightly different wine. A good winemaker has a passion for a specific expression of a specific grape or blend of grapes. That's what you want to try.
When you find your tastes agreeing with a particular winemaker's tastes, then you know you can pretty much trust what that winery is putting out, and it will justify the few extra dollars a boutique wine usually costs.
The way to find interesting boutique wines is to go to a good wine shop and talk to the owner or salesperson and ask them what they think expresses the passion of the winemaker. Mind you, choose an area of a country that makes the type of wine you like, decide on what you want to spend, and explore.