Before ording Dominus Est online, you might consider going to your local Catholic bookstore and asking them to order it for you. When they do this, they will probably order a small number to stock on the shelf to get it at a reduced rate. This will expose others to the book when they are browsing. It's probably a trade-off for what you will pay at a bookstore, versus what you would pay in shipping if bought online (even if it is not the most economical choice, consider the exposure you will give the book). Bookstores will need to know that the publisher is Newman House Press. Print out the form and encourage them to call for their rate.
If you must purchase it online, you can of course get it at Amazon, but I have only seen it being sold used for over $30, which is outrageous. Apparently, there is a market for it and people do not know of other online sources offering it cheaper. Compare that to the $7 price (NEW) here. Those in the UK can find it at Gracewing for £5.99