A new generation of sites works with a large number of retailers. The most relevant of those with a global orientation is probably Wine-searcher.com, based in New Zealand. It lists over 4 million offers from more than 19,000 retailers across the globe. It is fairly simple to use and the pricing information is transparent. Typing in 'Moss Wood 2008', as an example, lists 63 entries. An annoying factor is that only the pro version for US$30 will list them all.
An alternative is Globalwinestocks.com, which is based in Canada. It is of similar size and operates in a similar manner. You click on the wine you want to buy and get switched to the online site of the retailer.
This method can be efficient, but does, in my view, not fully replace the personal service a specialist retailer or the winery direct can provide. No doubt, it will work for some.