Hi, I have been riding around the whole area for a couple of years, and I can tell you in Belgium you dont need any maps of cycling routes, they are posted everywhere. Once you find a blue sign with a number and a picture of a bike follow it till you get to a path crossing (junction) there you will find other connecting paths on a posted map.
From Aachen follow the N 278 to Maastricht, +/- 30kms nice road with cycle path, there you can get on the Albert canal. Do not go to Liege, Belgium from Aachen you will not like it, real bad roads and serious hills.
If you realy want to buy one, every tourist office has them, Achen, and Vaals, The Netherlands are your best bets, get the cheap ones (just as good) you dont need much detail especialy if your taking the Albert canal.
Also the Albert canal is not the best place to cycle only a few sections are nice, too many factories and power stations on it, its good only if youre in a hurry. if youre lucky enough to get a tail wind youll get to the coast in a day with time to spare.
Good luck and safe ride.