An almond syrup substitute will do. I've been looking around a lot and can't seem to find where to get this. I'd like to use it in a Mai Tai and would rather not make my own orgeat syrup...
Kalustyan's carries BG Reynolds brand (no artificial flavoring), and I've seen it at Whole Foods Columbus Circle too. (BG Reynolds brand used to be called Trader Tiki.)
Definitely at the Whole Foods on Bowery too. I hadn't been in the beer store in a while, and they've expanded their selection of bitters and syrups dramatically.