The Snappa Flash brand are tied using Gamakatsu hooks. They come in sizes from 3/0 or the way to 8/0 , but I believe they only use J hooks. I prefer circle hooks on my flashers . Think I seen them in fishing, camping and outdoors. 

When I am to lazy to tie them, I buy most of my flasher rigs from Uncle who also uses Gamakatsu hooks. 

What is the red line that is used to hold the flasher material to the hook?

The red line is fly tying cotton/thread. It comes in dozens of different strengths and styles.

How is it tied off in the end? Or is it just glued in place?

http://www.youtube.com/watch?v=IlGtnAu7ejQ

What is the best place to purchase materials from? > glue, red line etc.

Any fishing shop with a decent fly tying section.  Flyshop has a good selection.

http://www.flyshop.co.nz/category/fly-tying.html

Although tackle tactics probably has all that you need for tying flasher rigs.

What is the best method of tying the line with the branches out to the sides?

I don't knot. 

The easiest is just to tie a loop using two or three overhead knots then cut the loop.  Would want to use fairly heavy mono because the overhead is a weak knot.... 

Another method, which is probably stronger, is to tie independent sections of line on using a surgeon or similar knot. That way if the dropper fails the whole rig does not break....  It also allows the use of different strength of line, for example the main backbone could be much heavier then the line use for the droppers. 

Many people use a dropper knot, but I am to uncoordinated to tie them properly. 

Does the line need to be snelled around the hook? And is the non slip snell the best for this?

Does not need to be snelled, although the snell knot is a very strong and simple knot and they might improve hook up percentages using circle hooks.