When you get to a dealer, see if he knows someone that can get you your drug of choice. (He might even have it himself.)
For me, this is one of the key strengths of Dutch "soft" (i.e. weed/hash) drugs policy: because it has been regulated, there's a strict line between the relatively harmless weed and the stronger "hard" drugs like XTC, heroin and the likes. Availability of the latter in the so-called coffee-shops over here is non-existant, which means crossing over from weed to the serious stuff also means a whole change of setting for acquiring it, as opposed to 'just' getting it from the same dealer.
Truth is free, but information costs.