If you don't have flaxseed meal, you can very easily make your own by grinding up flaxseeds in a blender. I usually grind up only enough to use for around a week or two because the oils in flaxseed can go rancid quickly once the flaxseeds are ground (that's why I never buy pre-ground flaxseeds).
Pre-heat a skillet to medium high heat, and your oven to around 350ºF. If you have a convection setting, use it.
Mix together the flaxseed meal, water, and a pinch of salt quickly in the preheated skillet, using a spatula to combine it well, and press it against the skillet for around a minute or so.
Push the mixture together into a ball of dough.
Roll the dough between two sheets of parchment or wax paper until very thin. You want the dough to be as thin as possible. Cut it into circles using large circular cookie cutters or using a pizza cutter to cut the dough into circular shapes.
Drape the dough circles over the bars of your oven grill. I draped mine over two bars and left a bar or two between each shell, but if the bars of your grill are very close together, you may choose to drape yours over more bars to make them wider.
Bake for 8-12 minutes until crispy. If the dough wasn't rolled thinly enough, it will have a tendency to balloon up and will take longer to get crispy, but should still be usable. Remove from the oven and fill with the ingredients of your choice. Note: The tacos in the picture are mini 4-inch tacos, and I was able to make 10 mini taco shells with 1/2 cup of ground flaxseeds. You can instead choose to bake these flat into homemade tortilla chips.