Here's what Wikipedia says:

Rayon is a synthetic filament fiber. It was the first artificial fiber to be produced. Rayon is the fiber that is produced from solublising cellulose. This can be done through nitration, to produce cellulose nitrate (or nitrocellulose), through reaction with acetic acid to produce cellulose acetate, or reaction with Scweitzer's reagent to produce a copper ion ligno-cellulose. All of these techniques involve partially substituting bulky groups onto the pyranose monomer units where there are hydroxy groups. This disperses the polymer and decreases intermolecular bonding, making it easier for solvents like water to enter. Once solvated, the polymer is extruded through small holes to produce the fiber, and bathed in sulphuric acid to effectively recrystallise it from solution. Today, most rayon is created by the viscose process. Wood pulp is mixed with concentrated sodium hydroxide, then reacted with carbon disulphide. The liquid solution is then pushed through holes in a device called a spinneret. As the liquid comes into contact with acid it solidifies into rayon fiber. Three common hole shapes in the spinneret determine the quality of the yarn: semi-dull, dull, or bright.

Rayon was originally named artificial silk or wood silk. The name rayon was created in 1924. Unlike nylon, rayon absorbs water, making it more comfortable to wear as a clothing textile.

As well as viscose rayon, there are other types such as anitrocellulose rayon, developed in 1891.

Nothing online, but if you're in the Bay Area, check Discount Fabrics on 4th St and the one on Mission. They often have LOTS of cheapity cheap rayon-y stuff.

And always pre-wash. It does shrink quite a bit.