On my shimano SLX 12 speed 1x direct mount crankset, there are 8 tiny bolts that fasten the chainring to the direct mount spider. They are some weird allen head with a post in the middle, and very small. Where may I find these?
As MaplePanda correctly points out, these bolts are not meant to be disassembled by the end-user. From Shimano's technical documents website, here are the pertinent documents related to your SM-CRM75 direct mount chainring. None of the documents refer this area of the chainring. This is a fairly gross oversight on Shimano's part, especially when considering the bolts were inadequately torqued from the factory and came out on their own.
The following links do not represent a product endorsement but rather are examples of hits I got when entering a search query into Google. That said, the type of bolt is called a "security," "tamper resistant," "pin hex security," or "pin-in-hex" screw or bolt. All describe the design of the bolt you show in your pics. Here's an example. Add the phrase, "button head..." before any of the aforementioned words and you'll have down the language most any hardware salesperson can use to find the appropriate part. If you take some calipers and measure the major diameter of the threads (the outer most diameter of the threaded part), then the length of the bolt below the cap (the length of the shaft not including the cap) you'll then have two more parameters with which to inquire for a correct piece. Sizing wise, a final parameter you'll require is thread pitch--the distance between threads--and the best way to get that is via a thread pitch gauge.
A couple notes: Shimano most certainly has spec'd metric size threads, probably M4 or M5 (I'm speculating here). The major diameter of the threads will be around 10-15 hundredths less than the whole number size. In other words, an M5 bolt's major diameter will be 4.85mm when gauged with a caliper, so you'll round up to get the correct "M" size. One thing that's gonna be a problem for ease of finding a workable solution is that area below the head of the bolt. That's either a thick washer/spacer or a machined part of the bolt termed a "shoulder." It may be difficult to find an exact matching bolt because of that damn shoulder. You can duplicate this by an appropriately sized spacer or even a washer stack (2-3 thin washers stacked to equivalent thickness of the shoulder). The washers or spacer obviously needs to be the correct diameter as well.
Here's a link to the type of tool--a pin hex security key wrench that will be required to work with these bolts. They also make kits containing security bits of differing sizes and shapes, like torx, hex, etc. These interface with a driver or socket wrench with appropriate fitting (or a 1/4 inch socket). As I'm writing this I'm realizing that you don't necessarily have to replace the security head bolts with security ones. You can also utilize appropriately sized button head fasteners as long as you can get them torqued to where they won't again come out.
All that detail aside, if your chainring is less than a year old and you purchased it from a place that is certified to sell Shimano parts, I would inquire about a warranty replacement because that happening is obviously a flaw in "design or workmanship" that led to those bolts falling out. Coupled with the fact that Shimano omits them from the technical documents--inferring no end-user maintenance possible--gives you a strong case for replacement. Beware they want to have the part to make their decision, so you'll still need a replacement while the warranty stuff gets worked out.