What shops do people go to / use when buying laptops for QubesOS?
ebay (search Qubes). You can take the HCL (hardware compatibility list) and shop anywhere (Craig list if you like face to face and test things before you buy). Just keep in mind that HCL does not protect you against BIOS and HARDWARE back-doors. Keep safe!
Check out System76 laptops in the HCL. They are assembled by hand in Colorado, which mitigates whatever risk you might associate with internationally assembled laptops (the computer parts, of course, are made internationally). Older laptops have the Intel ME neutralized as much as possible.
Nitrokey makes the NitroPad laptops which can be very secure if you trust their assembly.
Got my Lenovo Thinkpad X230 cheap on Lazada, got a T450S on the way
Paid cash on delivery, these things work very well in Asia!
Ebay should have lots to chose from, just takes a while to compare what is on offer & the HCL list - but that pays off.