Been building and upgrading my own DAW computers for years, I know what I need and where to find it. My dad wants me to order him a new PC, it's not for any power usage, just general use (web, Office) and image archiving. an i5 (or AMD equivalent) is good enough for him, as long as it lasts. Definitely not going to get a discount PC with a bunch of bloatware that will just give problems. What's the best source to buy a non-DAW barebones PC these days?

I don't see a lot of "bloatware" on new PC's.  I've installed several new PC's for clients in the last year and none had the amount of unwanted/unneeded software that used to come with PC's. I've installed Dell's, HP's, Toshiba's and Lenovo's.
 
But, www.newegg.com  would be a good place to start for barebones systems.

Just delete any bloat ware.

Newegg.com and magicmicro.com are the two that come to mind for barebones machines.

If not mistaken, any pc you buy directly from microsoft store is strictly bloat free. 

mudgel
Just delete any bloat ware.

FWIW, That's not as clean as those items never having been installed.  
Many folks don't know how to perform a full manual uninstall (including removing registry entries).

If you're comfortable building PCs, I'd build him a nice clean/quiet custom machine.
You don't need to go expensive.
It would be exactly what he wants/needs... and nothing more/less.
 

https://pcpartpicker.com/list/
 

I always wanted to try one of these Intel NUC's for a small footprint bare bones system.  Not the cheapest, but surely the most compact.
 
Just add RAM, SSD, and Windows.  Use the VESA mount to slap it to the back of a monitor, for an all-in-one. 
 
https://www.newegg.com/Product/ProductList.aspx?Description=intel%20nuc&Submit=ENE

abacab
I always wanted to try one of these Intel NUC's for a small footprint bare bones system.  Not the cheapest, but surely the most compact.
 
Just add RAM, SSD, and Windows.  Use the VESA mount to slap it to the back of a monitor, for an all-in-one. 
 
https://www.newegg.com/Product/ProductList.aspx?Description=intel%20nuc&Submit=ENE

Essentially the equivalent of a laptop...