Pool filter sand is a very practical and effective aquarium substrate, and it's dirt cheap compared to actual "aquarium" substrate at the LFS. The same is true for many other items that are useful to the DIY crowd. I have several sumps and other pieces of gear that came from Home Depot; most of the larger pieces are labelled Rubbermaid; in my previous home I had many hundreds of pounds of red/black fine gravel self-collected from a beach on the north coast of Lake Superior. Think outside the box; buying stuff from the LFS is almost guaranteed to be the most expensive way to get anything for the hobby. They only way I'll buy there is if I need something that can't be gotten any other way.
By the way, if you are planning on a planted tank that isn't planted...i.e. all the plants are merely tied or attache to bases, or floating...you don't need to use any substrate at all. You can also have a few plants in individual pots or containers, leaving the rest of the tank bottom substrate-free or at most with just a think scattering of substrate.
Filter floss is a special case. While you can buy it in bulk from other sources, it's difficult to be certain that it hasn't been treated in some way that would make it dangerous for aquarium use. Fortunately...you don't need floss at all, from anywhere! It's generally considered disposable, single-use only...although I know at least one cheapskate who has tried to clean it and re-use it in the past

...but you can buy good quality open-cell foam, cut it to the appropriate size for your filters, and then just keep cleaning it and re-using it. I use a product called Poret foam, available from Swiss Tropicals in the U.S. This stuff is not cheap, but I have Poret foam that is many years old and has survived many hundreds of strenuous rinses with no deterioration. You can buy it in blocks to make your own sponge filters, and also in sheets of varying thicknesses and porosities to use as both mechanical and biological filters in your filters. Buy once, cry once.
I like to have several sets of Poret for each filter, so that I can just take the dirty one out, drop in a clean one, and then rinse the dirty ones at my leisure. Typically, you only need to clean the topmost layer...which does the bulk of the mechanical filtration...while the other layers further "downstream" act as biofilters and can go many weeks or months without attention. Treated this way, Poret will pay for itself within a few months, after which it's "free", at least compared to constantly buying mountains of filter floss. It's also more acceptable from an ecological perspective; reduce, reuse, recycle.