I've been reading the PHB and haven't seen anything that lists the prerequisites for feats. Is there something that lists the prerequisites for each feat anywhere?
Feats only have prerequisites if they say they do
Not every feat has a pre-requisite. Here is the relevant text from the PHB (p. 165):
You must meet any prerequisite specified in a feat to take that feat. If you ever lose a feat’s prerequisite, you can’t use that feat until you regain the prerequisite. For example, the Grappler feat requires you to have a Strength of 13 or higher. If your Strength is reduced below 13 somehow — perhaps by a withering curse — you can’t benefit from the Grappler feat until your Strength is restored.
This doesn't explicitly call out that not every feat has a pre-req, but it says "any prerequisite specified"—if there aren't any specified, then there are none.
Sample Feat 1
Crossbow Expert [PHB p. 165]
Thanks to extensive practice with the crossbow, you gain the following benefits:
You ignore the loading [...].
Being within 5 feet of a hostile creature [...].
When you use the Attack action and attack with a one-handed weapon, [...].
Clearly, no prerequisites are indicated to take the Crossbow Expert feat, as none are mentioned anywhere. However, it is recommended to have crossbow proficiency to take advantage of the first and third benefits...
Sample Feat 2
Defensive Duelist [PHB p. 165]
Prerequisite: Dexterity 13 or higher
When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, [...].
Clearly, right after the feat name (Defensive Duelist), there is a prerequisite called out, in italics, listing a Dex requirement. If you don't meet it, you can't take the feat, and if your Dex drops below 13 while you have the feat, you can't make use of it.
Not every feat has a prerequisite, and the default assumption in absence of one listed is "none," because no other default makes sense. Defaulting to, say, Str 12 is nonsensical for feats like "Observant." We can extend this logic to any decent pre-req assumption and conclude that there is no pre-req by default.
Note: I patterned the display of the feats off of the PHB; this is quite close to how they actually appear; in general, if there is a pre-requisite, it appears in italics following the name of the feat. AFAIK, there is not yet a compiled list of which feats have pre-reqs and what those pre-reqs are.
D&D Beyond, sort of
While D&D Beyond's feat list doesn't list the prerequisites for each feat, it lists all published feats (and potentially future playtest ones) and will let you filter to specific prerequisites, such as a Dexterity requirement or being a Dragonborn. Interestingly (as of writing this), it does not have a filter option for 'no prerequisite'.
Note also that only the Grappler feat is part of the SRD, and so you will need access to the source (either on D&D Beyond or your own book) for you to get the full text of the feat. Otherwise, only a summary of the feat is provided in the "Notes" column of the feat list.