Can I purchase a catalytic converter on line and have it instaled at a shop?

Yes, but in my experience, most shops shy away from installing parts that have bought elsewhere b/c they can't give a guarantee on the work if they don't use their own parts. If you can do the labor yourself or have a friend that can do it for you, get the part yourself. Otherwise bite the bullet and have a shop do it all.

get a factory converter or bosal. they are the only ones that will keep the light out. buying aftermarkets are cheap but they never work and you cannot get a refund. thats why the factory one is 750-800 and the others are 200-300. you get what you pay for. anyway, if the shops install them, there is no warranty expressed or implied. I have people sign a waiver to this fact before we do the work. Roy