You might try http://www.LJOLeather.com I have purchased items from them at the show here locally. They are VERY nice people and seem to do a lot of research in selecting the products they are willing to sell. I had forgotten about them when you asked earlier. I hate buying online, so I have hesitated to order extra lengths of rope. They have cotton, manila and some sisal too. You might call them as I know they have more stuff than what is on the website. The man told me that himself.
On a side note, my birds, the amazon and the grey seem to prefer a rope they can wrap their feet around, maybe provides a safe grip for acrobatics?? I was also looking for something extra thick, but now I am unsure about that. I might have the LJO guy bring me samples to the show this weekend...and I can report back.
oops...I think that show is the 29th.