Back when I was dirt broke and the nearest hobby store was a 6 hour bike ride from my house (before I had a car...which brought it to a more reasonable 45 minute drive)...
"Flock" is a bit non-specific, but there are a few different recipes for it.
The first involves using sawdust. Alcohol dyes work great for coloring it, get a big bucket and a bottle of dye. Wood working and art stores sell these. Mix it up according to directions on the package. Mix in your saw dust (softwood saw dust works best - generally can get bushels of it for free from construction sites near their chop saw. Stir the slurry of dye and saw dust until everything is evenly saturated. Reach in and grab a handful (wear gloves...the alcohol dyes are permanent and will stain you hands an interesting shade of green - or whatever other color - until the outer layer of dermis is shed). Squeeze out all the liquid, or at least as much as you can feasibly squeeze. Spread the dyed sawdust out on a framed screen (you can make your own or generally find cheap ones from home centers). Let it dry and than pack away. Normally you can make a few 5 gallon buckets worth of flock in an afternoon for around $20 (several different colors worth of dye as well as other consumables like gloves and stuff).
Ground foam is the other big one that is referred to as flock by some. You can get cheap uncolored sponges from discount stores. To color these use fabric dyes. Again, mix it up according to directions. Dunk the sponges into the dye mix and saturate them fully. Do not squeeze them out too much - you want to allow the dye to soak in. Cut the sponges into 1 or 2 inch squares. Feed the cut squares into a meat grinder (I used a cheap manual one I bought at a garage sale for under a dollar). The meat grinder does the work - you just choose the size of screen you want to use. To get even finer foam - you can regrind it as well. Again - spread on the framed screen and let dry.
To add some depth to the ground foam, you can dye it again after the grinding process - or use thinned paint which won't soak through as completely and tumble it.
Be careful with the foam from upholstery shops - much of the newer foams used for upholstery are entirely synthetic and do not take color well (or at all). Sponges work great because they are generally a natural material (either real sponges or a manufactured cellulose based sponge).
Now though...unless I need a very specific color...I buy my stuff from Scenic Express. They are cheap, sell in bulk and provide good service. If you want more details on it - you can generally find some good info from some of the penny pinchers on model railroad sites.