What kind of fuel do I use for a Hario TCA/50A coffee syphon?
Ask them for Brennspiritus. You can get it in bigger drugstores like DM or Rossmann, I don't know who else will have it, but maybe home maintenance (?) stores (Baumärkte). 

You can't use butane, that's a different fuel and obviously not suitable for your case. You should be able to use pure ethanol, but due to alcohol import laws, medicinal ethanol without dilutants is extremely expensive, and medicinal ethanol with dilutants might have dilutants which create something bad when burned. Brennspiritus is ethanol with dilutants which can be burned safely, while still making it undrinkable. 

If everything else fails, you can start burning high-percentage drinking alcohol, my grandpa uses it sometimes on Spiritus burners. The problem (besides being more expensive) is that you need both a high ABV (something around 70% will be good) and a pure alcohol (vodka or obstwasser); alcohol types which you are likely to find at these percentages (such as rum) can leave layers of badly combusted gunk inside the burner.