Sut hwyl, Pibydd?
I'm afraid I don't know of any particular sites for Welsh dance tunes - although someone here probably will! I'm west Wales based, but I hear that there are good sessions in the Nelson, in Bangor. Get on the Trac mailing list (firstname.lastname@example.org) and they may be able to put you in touch with likely suspects. I know there are a couple of harpers in Wrexham (father and son) but can't remember their names. The father is a chiropodist, of all things... so presumably should know all about dancing!
I understand that Ceri Matthews, here in the west, has done a lot of work on creating a Welsh repetoire for pipes. I can't remember if he's published anything or not...