Dirndles and Lederhosen are traditional wear in Bavaria so you can find places to buy them anywhere within Bavaria.
Good Dirndles and Lederhosen are very expensive (just like good kilts are expensive in Scotland). Jo's suggestion to seek out second hand stores is probably a great idea if you can find something. The more attention you pay to the costume, the more you can tell the difference between quality traditional dress and cheap costume.
I think Dirndles are extremely attractive. Especially on Bavarian women. I like the longer Lederhosen but I couldn't see wearing them to Octoberfest to get doused with beer (or worse).
I saw a lot of young German men wearing a modern interpretation of the traditional garb to Octoberfest along the line of a pair of shorts or capris, a beer themed t-shirt (Hofbrauhaus, Lowenbrau), suspenders, hiking boots, knee high wool socks and maybe the traditional hat and sweater.