You can buy them only at a privately owned "Beer, Wine, Deli" (so-called because any such establishment is required to sell food as well as alcohol) or at county-run liquor stores, which are the sole vendors of hard liquor and are closed on Sundays. I find the system is a bit of an annoyance and certainly not conducive to bargains. For some people, though, it's a much bigger deal.
A Washington Post article yesterday noted Montgomery is the only county in the U.S. that controls both the distribution and sale of all alcoholic beverages (to the tune of $191 million last year). That system has created a cumbersome bureaucracy for restaurateurs in the county, especially when it comes to ordering high-end labels to offer their customers. In addition, the county adds 25 percent to the distributor's wholesale markup.
According to the article, the operations chief for the county's Department of Liquor Control plans to meet with chefs and restaurant owners to hear their concerns. In the meantime, you know where to go for a bottle of plonk at a high price.