Himeya shop used to have a rather large selection of doujin music (primarily touhou stuff, but I saw some Key stuff there too if I remember correctly), but for some reason they stopped carrying all of it. I'd imagine you could request something in particular through their request feature, but I don't know that that would work well if you're just looking for different things to sample.

I know for a fact that a lot of doujin music is sold at conventions (I was able to pick up a couple dobu usagi albums and a final fantasy metal arrange album at otakon), so you could always try that if its a viable option for you.