WAB is currently out of production, after
GW decided to close its Warhammer Historical section (something about not being to copyright Romans...

). You can still find people playing it, however, since a simple thing like the discontinuation of a ruleset or a new edition isn't going to stop Historical wargamers.
As such, the only way you're going to get the ruleset is by buying them second hand, or having someone photocopy their current copy.
Two rulesets, however, have been acknowledged as WAB's successors - 'War and Conquest' and 'Clash of Empire'.
YMMV on whichever one is better. I happen to have a copy of CoE myself, but I haven't gotten around to doing anything with it. Great Escape Games is actually on Dakka, so you can always
PM them a question or two.
Also, this would probably do better down in, you know, the Historical section.