Since the subject of the sentence, my availability, is singular, you need to use is. The phrase for the next two weeks is just additional information describing my availability. It should have no bearing whatsoever on your choice of whether to use a singular or plural verb. That kind of choice is always governed by the actual subject of the sentence. Take a look at the following examples:
That book over there with yellow pages is mine.
The subject is singular, so the main verb of the sentence should also be singular in form.
The dogs that live in the red house across the street are not there.
The subject is plural, so the main verb of the sentence should also be plural in form.
So, use is:
My availability for the next two weeks is as follows...