Here is some of the least inexpensive rope I have found. Cajun Ropes » Cajun XLE
They have different types but the XLE is the "high" quality stuff. For your sheets I would do either 1/4 or 5/16. On my Lido 14 I opted for 5/16 because it is easier on the hands and 1/4 felt a little too small to me. For all other lines I went with 1/4". I have received the lines and am Very happy with them. When I ordered them it took a couple weeks for him to respond so don't get frustrated if it takes some time.
Edit: You could also get away with the Dinghy braid but they only offer it in 1/4 as the biggest size.