ABC tune collections
There is a pretty comprehensive list of ABC tune collections at
One of the best collections of English historical manuscripts is at
The Village Music Project.
For a mixture of nationalities and styles, a commonly used starting point is at
The Fiddler's Companion by Andrew Kuntz.
For a selection with an Irish bias, you can go to The Ceolas tune archive.
For a collation of favourite English session tunes, see The Lewes club list.
There is a similar list with an Irish slant at The Session.
And for a French list, try Le Session.