The reason for the existence of the "Where available" field:
When this field was not available, people just set Availability to "Available 2018".
If you want to buy a device, what would be your next question? Right, "it is available, so where can I buy it?".
Simple answer: Just google it.
That works for many devices, but from my own experience I find it extremely difficult to find good information on availability of asian devices (amongst others). This means that that the correctness of "Available 2018" can not be checked, and the user who wants to buy this device is left alone, with his question "so where can I buy it?" staying unanswered.
The "Where available" field has then been introduced in order to make the "Available 2018" more sound and verifiable, hence making it more useful for the user who wants to buy the device, and easier to manage / update for the wiki admin.
Why only/mostly amazon?
Because amazon is (most times) the first that pops up in a google search. If it is available on amazon, it is widely available.
If you see the Availability field from my (wiki admin) perspective: It is highly labour intensive to keep this field updated and useful for the users.
When I'm creating a new dataentry, I do what a user would do who wants to buy this device: google the modelname of that device. If amazon pops up at first position, I'm done. Why invest more time, if the device is widely available and the user can easily buy it? Next device, next dataentry please.
Imagine the number of availability fields to be kept updated: Currently 385 "available" + 285 "unkown". Assume 1 minute per check and you have roughly 11hours of fun with the search engine of your choice. If it is you who does the update, you are happy when it is available on amazon, because that saves you 10mins with google translate, trying to understand japanese or chinese webpages, and failing miserably because the text is rendered as an image - untranslatable for google. BTDT, more than once.
Last but not least: Because wikidevi does it (example -> databox on the right side of the page). You mostly find amazon links. Easy to manage, widely available.
Why no categories?
Too unflexible, too unspecific. In order to be useful for the user, specific, concrete answers are prefered over unflexible categories which do not allow me to enter e.g. "mouser.de, mouser.com" (not digikey, not farnell, not RS nor any other big player of the category "Electronic Components Sellers").
Feel free to add more than just amazon, but keep in mind that the "Where available" field is not intended to be an exhaustive listing of all possible sources; it shall be useful for the user who wants to buy the device, and make it easier for the wiki admin to verify the correctness of the availability status each year.