Moss, I suggest that you buy a large rope from the beggining. The cost of buying plus handling and delivering a short piece of rope is the same and in a couple of weeks you will be ready to go higher and you will have to pay again to get the longer rope.
www.wesspur.comhas rope pieces in odd sizes that they have on sale if you want to go that way. but You can get 150 ft of the Fly rope for $89.00 plus delivery charges and that is the top rope these days from what I heard from the experts here and on the buzz site. I will be getting my Fly rope soon since my basic rope is almost one year old and showing some damage from the regular climbing use.
Good luck and also buy yourself the Tree Climbers Companion by Jepson. It has all the info you need and much more than the smaller book from Flowers

Take a class if you have a chance also!