Turkishvisa.com

You purchase them right at the airport. Be sure you have an American twenty with you.

I have no experience with that website, but the official site (https://www.evisa.gov.tr/en/) could not be cheaper or easier to use. You fill out your information and use a credit card to pay $20 and in a few days (mine came the next day) they send you an email with a link you can use to download your visa as a PDF file. I don't see any benefits to using turkishvisa.com personally.

Yes, Erik. That's exactly what my son did to get our two visas for our upcoming trip to Istanbul. Could not have been easier. On my previous trip to Turkey (2013), we did pay with dollars as we entered the country but there have been mixed posts regarding whether or not you are still able to do this.

Use the official https://www.evisa.gov.tr/en/ web site; it's easy. Turkey no longer wants to take cash at the border, although they still seem to make some exceptions.

I am going to Istanbul in April this year and got my evisa at https://www.evisa.gov.tr/en/.
It was a very simple and easy procedure and I had my visa issued within minutes and sent as a pdf for printing.
It cost $20.70. Good for multiple entries and valid for 90 days from date of arrival.

If you do not arrive with an evisa, as of last July, United States citizens pay $30.00. This may be old news already and you might be expected to have an evisa.

This is an addendum to Pam's comment: I landed in Istanbul this past September and was able to purchase a visa stamp at the airport. As she says, the price has risen to $30 which is more than the web price. I carried some American banknotes just to pay for the visa. A bonus to some visitors is that you get a large stamp, like a postage stamp, attached to a page within your passport. It seems that the web purchase you get a piece of paper which you have to manage separately (I believe that the web site says that if you misplace it you have to buy another visa).