They are difficult to find. I know of Americancomputech.com that you can use as a reference, but WHATEVER YOU DO, DO NOT PURCHASE FROM THEM! Use them as a reference only. They have a horrible record in all departments (broken computers shipped, takes 2 months to ship, no 800 number, help desk barely speaks english, wont reply to emails, help phones dont answer, basically No customer service, etc...the list goes on and on.)
But they are a decent place to get an idea of how much a computer will cost you, if thats all you're looking for.