where do i get an arbor/trellis for over my garage?

Not sure what you mean by "over your garage", since "over" would mean you'd be on quite a tall ladder to do maintenance on whatever plants occupies the arbor.

Open the yellow pages phone book and start calling some real garden centers. While buying plants this spring, I must've seen 50 different kinds of trellises and arbors at various garden centers.

You can get custom-made trellises made by a carpenter, expect to pay. Or, you can build a trellis using a few tools (circular saw, square, tape rule, pencil) and wood of your choice/budget (white oak, PT lumber, cypress, redwood, teak, cedar). Make a drawing with dimensions. Take a look at existing plans. Applying a UV finish will make it last a lot longer. Sometimes a child will want to climb a trellis, unless you are growing roses on it.