Expandex is tapioca starch. You can buy this online or from Chinese grocers. You can likely replace it with other starches also. I'm in the UK so I haven't tried the brand you've bought, but can advise that buckwheat is usually subject to cross-contamination. It is usually grown in crop rotation with wheat, hence I haven't found a suitable buckwheat flour yet. My best advice for starting out is to buy a few kinds of flour (rice flour, sorghum, millet, etc) and mix your own blends. This gives you much control over the result. It is an investment at the beginning but can be cheaper overall, especially when buying online. I found this article which gives good advice about blending flours:
One ingredient that I use regularly is psyllium husk. It gives the dough a gelatinous quality similar to gluten. I would definitely strongly advise looking at recipes that contain this and use long proves like traditional bread. I also don't use eggs or dairy in my bread as the psyllium bind things without weighing the loaf down or adding fat. Please let me know if you've any questions about that! Good luck!