Especially if you see yourself using graduated filters in the future, I would recommend using square (rectangular) ones. 4x4 inch the most common industry standard. Somehow Lee managed to get it into still photographers' heads that they make the best filters, but 4x4 filters are also standard in the movie industry, where Tiffen and Schneideroptics are well regarded. In Germany, for example, there are countless professional production rental companies that put filters up for sale with just micro scratches on the sides, nothing you would see in photographs, for 30-50 USD a piece. I would assume that similar deals can be had in the U.S. Well worth a look! Also the filter holders (matte boxes) from arri, and schneider come up for cheap on ebay from time to time, quality on these is phenomenal, compared with the Cokin and Lee plastic mounts.