Best answer: The ASUS store. The ASUS ZenFone 8 is only sold directly by ASUS's online store. You can order the phone with either 128GB of storage for $599 or 256GB for $699, and ASUS will provide free FedEx Ground shipping to the 48 contiguous states.
Your only option, really
For now, the only place you can buy the ASUS ZenFone 8 for retail is ASUS's own online store. ASUS doesn't have a huge presence in the US smartphone market. So, it's no surprise that the new ZenFone 8 doesn't have as prominent of placement in major retailers or mobile carrier stores as the latest iPhones or Android phones.
That said, it may just be a matter of time before some more options begin to open up. The previous ASUS ZenFone 7 Pro (global edition) made its way to Amazon, so the ASUS ZenFone 8 may just need a little more time to reach additional retailers. Until then, the ASUS store remains the default option. So, if you want the ZenFone 8 now, you should go to the ASUS store.
If you don't want to go through the ASUS store for some reason, you can always go through eBay. While we haven't seen major retailers offering the ZenFone 8 through eBay, several sellers still list the phone through this online marketplace. That said, we haven't seen the ZenFone 8 listed on eBay at reasonable prices. Most listings have had the phone marked up $100 over retail price.
Also, when it comes to buying on eBay, there's a certain amount of risk involved. With marked-up pricing and uncertain sellers, you may be best buying it directly. If you do make the jump to eBay, be sure to do your homework on the seller.