First Pet Advice - Overwhelmed by everything out there

Our 11 year old son really wants a pet and after a lot of back and forth he settled on wanting a turtle (we cannot have any fur based animal in the house due to numerous allergies).

This is what we know so far:

• the turtle will live in his room
• we will get a 40 gallon tank for him to house the turtle (cannot do anything larger)
• obviously it needs to be very small (so I know the males are generally better for that)

So the questions I have are:

• best turtle given these limitations. I've read musk, terrapin, mud.... Not really sure which we should consider. Is one better as far as sociability? Care?
• Where do we get one? Do we really go to PetCo? (I looked on petfinder but there was nothing within 100 miles of us)
• Appreciate any advice on tanks, etc

I know a lot of this is already around in various threads, but trying to read through them was a bit overwhelming so I hope you don't mind me asking in a new thread.

And I know there will be some people that say not to get one because of our limitations. That boat has sailed.