You can buy it in standard lengths, typically 12-13 ft, or order custom lengths (& choice of color) from online sources. We have done both.
The round, 1/2" and 3/8" diameter models are pretty much a commodity item these days, so I don't think it makes much difference where you buy. Lowes. Home Depot stores, Amazon online, etc. An online shop called http://www.1000bulbs.com/category/rope-light/ offers a wide selection.
There are also flat (tape) strings that may be of interest to you as well. They are typically 12v but have 120vac adapters available from the same source. You buy them in fixed lengths and cut to fit yourself.