I shall dispense with an unnecessary description of each of these products and am sure I don't need to say that this site is in no way sponsored by any of the manufacturers, however this is a quick visual hint of what you might be looking for in the shops. All these products can be bought in easily accessible high street stores such as general supermarkets or heath food shops such as Holland and Barrett but you might need to wade through the dodgy supplements or stray down a hitherto unexplored aisle to find them! Goodness Direct is an on-line option that I have never used but seems to have an extensive selection. I've stolen a lot of their product photos so they deserve a mention!
Chia and Flax (or Linseed) share a useful property in that when soaked in a liquid they exude a sticky substance that can be utilised in cooking to provide binding properties, a bit like the role of egg in some recipes. Generally, soak 1 tablespoon of seeds to 3 tablespoons of liquid for 5 minutes to replace the given quantities of one egg. Water is fine but if it's a flavoured recipe you could use something else such as (cold) coffee or orange juice. Seeds can be ground or used whole but bear in mind they will be visible in the recipe so for an innocent looking sponge you might be best off using a commercial product.
In Fillings, Omelettes & Dips