Where to buy Oyster card

At any manned tube station, including the three at different terminals in Heathrow. The price is fixed, it doesn't change.
If you get the Heathrow Express (on which an Oyster card is not valid), then buy it at Paddington tube station.
When you buy it, you specify how much credit to add. You can add more credit at any manned tube station, at tube station ticket machines or at many convenience shops, look for an "Oyster" sticker on the door.
Whenever you use it the display shows how much credit you have left, but it only appears for a second, you have to be watching.

Can be a machine, too.

And when you leave London, you can cash out the Oyster if you don't think you will be using it again and get refunded on the spot for the remaining value stored on the card (has to be a reasonable amount like around 10 Pounds) and the 5 pound deposit. Just make sure that whenever you add funds to the Oyster as needed you do so using the same method: cash or exactly the same credit card. If you don't they will have to mail you a check which will be in pounds.

You could also look at possibly buying an Oyster Card where you live. Before we flew to London a couple years back, I purchased our Oyster cards here in Melbourne, Australia. It already was loaded with about 15 trips on it and it didn't cost us any extra than if we bought it in London. I did this as it was our first trip overseas and just wanted to make it easier for us once we landed in Heathrow, so we could just go straight to the Tube station and jump on, without the nervous hassle of also buying tickets.

There are no bargains for buying the Oyster card itself. It is simply an electronic wallet in which you deposit whatever money you think is appropriate. Each transport ride requires touching the pay pad on entry and exit; the fare is automatically
deducted. There is a maximum per day, so after a certain number of trips you are riding free. You are closer to that point on your first day if you use your Oyster for the fare into London. The fares vary by number of zones crossed. The deposit for the card is supposed to be refundable when you leave London. A simple Google search will dig up all the rules.
On a visit a few weeks ago I was struck by the number of transit officials stationed by the Oyster vending machines helping dumb tourists like me sort it all out. Very cheerful; very helpful; very much not like the churlish attendants 50 years ago.

"You could also look at possibly buying an Oyster Card where you live. Before we flew to London a couple years back, I purchased our Oyster cards here in Melbourne, Australia. It already was loaded with about 15 trips on it and it didn't cost us any extra than if we bought it in London."

That sounds great. Where can I buy them in the US that it's the same price as at a London ticket machine?

"Where can I buy them in the US that it's the same price as at a London ticket machine?"

You can't.

If you buy one in the US, it is shipped to you which costs money. It will also be a "visitor" Oyster which has various restrictions on what type of travel cards can be added, you can't register the card online, and you can't cash it out when you leave (if you had planned on doing that).

I can't really speak to the cost in Australia, but I went looking for an Oyster before one of my trips to London and was not able to find any except through a couple travel providers. They charged around $15 each to deliver in addition to the same cost you will pay at the ticket machine in London and picking them up in person for no additional charge was not an option.