It's poorly organized. You can see that it's alphabetized by the first name of the person who does the tests, not the name of the company or even the tester's last name. Also, depending on what you're viewing it on, it's not exactly obvious that there are three more pages of testers after this one. But at least it's a list.
The city recently made some changes on its website. Not sure if that is causing a problem with the link or not. It won't open for me today either.
Here's a link to a different City of Frisco page that also has information about backflow inspections, including a link the list of inspectors (which is the one currently not working; hopefully this is just temporary). However, scroll about 2/3 of the way down the page to the part entitled Backflow Testers and it also has a phone number that you can call to get a list. If you call, please tell them their link isn't working. Can't remember the exact cost, seems like it was maybe around $125 or $150.