If I want a Canon FD lens to Canon EF body and infinity focus How bad will it degrade the image quality?
Sorry, I have no idea because:
1) I have never used that type of adapter. (One benefit of shooting with a GH2 is that the short flange distance means I don't need adapters with optics.) (Which brings up another possibility -- have you checked how much a mirrorless camera owner might be willing to pay for these lenses? Perhaps you could auction them off and then use the money to buy a native EF lens.)
2) As Rorschach noted, the quality of the optics varies depending on the manufacturer.
If I buy a adapter without the optics for infinity focus. Will I keep image quality and lose zoom focus?
To be clear, you'll lose infinity
focus (that is: the ability to focus at/to infinity, regardless of the lens' focal length
) (and maybe more/closer than infinity, as well, again regardless of the lens' focal length), not "zoom" focus.
Do you perhaps know of a site that sells them at a reasonable price?
No, but a Google search should turn up several different possibilities.