What they are offering is actually very simple, and systematic.

While the list of foods containing fodmaps has changed a lot lately, there are several sources that are up to date. University of Michigan has a lot of reliable resources.

The system is described in full here: http://www.myginutrition.com/diet.html (U of M)

They also have a pinterest that has tons of ideas and specific brands they've tested... Which is here: https://www.pinterest.com/UMGIdietitians/

Full list of foods here: http://www.myginutrition.com/downloads/Low_FODMAP_foods.pdf

The warning my dietitian has given me, is that many of the websites out there are using out of date information pertaining to foods; and that I should only trust their lists.