Coir, also known as coconut fiber, is a natural product sold as a soil amendment and hydroponic growing medium. It is made from shredded coconut husks. It is lightweight, easy to work with, does not require special handling or protective clothing, and it can be easily composted. It has been used as a soil conditioner and growing medium for quite awhile, but is not as well known as some other options. It is structurally similar to rockwool, featuring long interwoven fibers that form a thick well aerated mat. But the fibers are much larger in diameter than rockwool.
Advantages are many. Unlike rockwool, the coconut fibers are non-irritating which makes it positively blissful to work with. It is a naturally occurring fiber, which many growers favor over manmade materials. Using coir doesn't add to the waste stream, but rather diverts it from the waste stream of the coconut milk processing. It can compress like rockwool, but offers a looser weave that can be torn apart, fluffed up, or compressed as needed. It offers good water and air movement. It can be packed tight enough to provide support for all but the smallest seedlings, yet larger seeds will find easy sprouting conditions. It can be purchased at many home improvement stores or gardening centers, even those that have no other hydroponic products in stock. It can be safely composted after use, or will simply slowly degrade outside. Best of all, coir is typically sold in tightly compacted bricks which are very easy to store until needed. The expansion rate is amazing - a single brick will expand to fill a wheelbarrow. Just add water and wait. One odd feature is that if you wanted something of a hybrid operation, where plants are started in hydroponic or aquaponic situations then planted out in soil later, this is one of the few growing media that gracefully allows that transfer.
It does have some disadvantages. While providing a woven texture like rock wool, it is not so tightly woven so it tends to dry out faster. The natural fibers will break down faster than just about any other growing medium, so you will be replacing it at least once a year, possibly more frequently. It is slightly acidic and it does react with nutrient solutions, so you will have to monitor pH and adjust accordingly. It will also slowly but regularly shed small dander-like pieces which can clog irrigation and drainage lines, so filtration of that material becomes very important very quickly. And finally, thanks to its relatively fast degradation, it cannot realistically be reused.
We've used coir extensively for a variety of horticultural reasons, primarily to lighten up heavy potting soil. It does that very nicely, while still retaining moisture like a sponge. We've also used it in hanging plant baskets, which lightens up the basket weight while giving the roots plenty of humid growing room. Additionally, we have occasionally used it as a sprouting medium for seeds, with varying amounts of success. We have lived in a number of different climates, some of which have been very dry and some of which have been very humid. Coir will hold moisture for amazing amounts of time in humid climates, almost too long if the plants are young and not drawing up a lot of water on their own. However, it will dry out a little faster in dry climates, so irrigation is still needed for young plants with new root systems. Finally, we have used coir mats as a short-term weed suppression material. It worked well for that purpose, and broke down easily enough after the season's end such that it simply melted into the planting bed soil. Bottom line, we like working with coir because it's relatively clean, easy to handle, not as dusty as some other soil amendments and it's a natural byproduct which would otherwise be sent to landfill.
Coir has become a more common gardening and landscaping item for both commercial and residential purposes in the last 10 years. Therefore it has become fairly commonplace in the Gardening section of the home improvement retailers, as well as with commercial nursery and greenhouse equipment suppliers.
The first likely source for finding it is as close as your local hardware store or home improvement center. Many such businesses regularly carry it, either during the spring growing season or year-round, in either compressed brick or compressed bale form. Either form can be used as either a soil amendment or as a pure planting medium.
A second source is the specialty landscaping supplier, nursery supplier or commercial greenhouse equipment supplier. If you live in a metro area, you will probably have at least a few such businesses within a few hours' drive. Even rural areas often have regional supply houses for that area's farming, greenhouse and nursery businesses. Those supply houses will most often deal with larger scale commercial clients but they may also work with walkup and small-scale buyers too.
A third possibility is a wide variety of online retailers. Given how much the fiber can compress, it can be shipped relatively cost-effectively. So online retailers can provide competitive pricing even compared to local suppliers. They may also feature local dealers who offer online pricing with local pickup. For instance, Gardener's Supply Company offers coir as a 12-pack of bricks, at a very competitive price.