I’d recommend Giant-Size X-Men #1, which started the Claremont era and is responsible for most of what we would think of as the X-Men today. I recently got Uncanny X-Men Omnibus Vol. 1 on loan from the library, which collects the first 30 or so issues. It’s pretty expensive (and rare, I believe) so you might want to look into going through your local library as well. Mine got it through the inter-library loan system, which is nationwide, your library might have inter-library service as well if you ask them about it.

It’s a bit hokey, with some mid-70s cheese, but it’s fun to see the start of what defines the X-Men even to this day, arguably (I don’t think many would object, really) more so than the Lee/Kirby originals. Although, I have been meaning to go back to the beginning when I get the chance, only because it’s my least read version of the X-Men (ironic, I know).

Also, there are less…legal means, if you don’t mind those avenues: Uncanny X-Men Omnibus Vol. 1-3 Download