More or less a closed shop for aways in terms of buying through the club mate. They're sold on past loyalty and the criteria you need to meet is extremely high. Unless you're a registered member / season ticket holder who attended around 10 or so aways during the previous season you probably won't have a sniff
As others have said, the home end is probably your best bet. Although with Brentford being newly promoted and a game against Liverpool being quite a spectacle in their eyes, the match may not go on general sale due to high demand and Brentford themselves may require that you to have certain criteria / history in order to buy.
Only other option would be the massively marked-up, touted tickets which will no doubt be knocking about online, but I wouldn't reccomend going down that route