For an ebook, you just buy it the same as anyone else.
For a KDP-print book, some authors have the possibility of ordering books at cost from under the three dots ... at the right of the Bookshelf. If it's not there, I'm afraid you have to buy just like anyone else (though you can always drop the price, especially if you want to order a lot, then raise it again). For whatever reason, only a few randomly selected KDP-print authors are allowed author copies at cost.
If you have a print book through CreateSpace, just order from your CS dashboard. Easy. Another reason to publish paperbacks with CS rather than KDP-print.
On your CS Dashboard, look to the right of your book title and click on the blue words reading Order Copies.
What could be easier?
I find that ordering ten books at a time gets the shipping down to under a dollar a copy, so the typical book costs me $4.25-$4.50. I use standard shipping. The books always reached me in three business days by UPS, though I understand that things were slow in the week running up to Christmas.
(Don't trust KDP to publish a print edition. Don't trust CreateSpace to publish an ebook. Each does one thing well and the other thing poorly.)