Jonah found a website that sells various religious clothing, including rope cinctures in Royal Guard colors.

They sell them for about $15.50 + shipping. They look very ice, and have neat little decortive knots at the ends of the rope. They come in 12 foot and 15 foot lengths. This seems a little long to me, at least for Royal Guard purposes. The rope is doubled over, so I'm thinking I would need 8-9 feet of rope at most.

Last week I stumbled across some burgandy rope in the fabric department of Walmart which I think will also work, for about $1.47 yard. Not as nice as the Almy cincture, but certainly workable. (I'll have to pick some up at a later date, I'm not sure they had enough at this particular store).
Anyway, Almy has a nice instruction page for How To Tie a Rope Cincture. It's very simple, but I'd have never guessed how on my own. You can view it here: www.almy.com/Cincture.pdf
I've also found a tutorial on how to tie those fancy nots at the end of the rope, called Barrel Knots. Barrel Knots are used in fishing and other activities, but the tutorial I found is by a friar specifically teaching how to tie it onto a cincture. It is found here: celticmonk.tripod.com/cinctureknots1.pdf
I haven't tried tying one myself, but I will after I pick up some rope at Walmart.