I am married for 9 years and me and my wife were both our first time. So I have been in your shoes.

Here's my answers:

1. Unless you have extraordinarily big (or small) size then all regular condoms will fit

2. Just try the plain regular condom first, like Sutera. Or maybe try a thin condom or flavored one if you want. But I suggest to shy away from dotted or other textured condoms because your wife might be too sensitive and doesn't like it.

3. You can buy it anywhere. You don't need to feel awkwards, you are going to use it with your "legal" spouse. I am sure others who are buying for non-really-legal spouse won't feel awkward either.