When using roof anchors, the first step is to make sure they are OSHA/ANSI certified. The next step is to figure out if you want temporary or permanent mount. A good online store to look at a lot of different overpriced models is
www.grainer.com. For most anchors, the instructions "require" attaching the anchor with 16d nails to one of the roof rafters regardless if they are permanent or temporary. Of course, there are exceptions like the anchor for standing seam metal roofs and those that require a section of roof be removed so they can use a bigger gripping surface against the sheathing so you don't have to use the nails.
OSHA/ANSI doesn't care if you have to remove shingles or ridge vents to install the anchors, however there are other options if you don't want to get into all that work. The first option is to use roof brackets. Then there are multiple ridge hook systems that can be attached to the ladder, the roof itself, or like the ultimate ridge hook be able to go on top of the vent. It needs noted that some of these systems, such as the ultimate ridge hook, are not meant to use as actual anchors for fall protection but rather as anchors for placing or building scaffolding systems.
For purchasing, check your local Home Depot, Lowes, Menards, and some wildlife control supply stores like WCS. The anchors may be part of a fall protection kit or sold separately. For instance, Home Depot sells a temporary anchor for $15 that is sold through Grainger at $65. ABC is offering a "new" permanent anchor in their catalog this year that can be installed anywhere under asphalt shingles (in three colors), but it does stick up from the roof unlike the permanent bands with D rings on either side that can be mounted under a ridge vent.