I need a new laptop. Any suggestions for a good laptop that runs well with Elementary? Anywhere to purchase one with Elementary installed? or do I need to buy a Windows machine and dual boot?
You don't need to dual boot with Windows. Just burn elementary to a USB using etcher or rufus and boot from the USB. Install elementary and choose to wipe the disc rather than install alongside.
I have a Dell Inspiron laptop and have never had any issues with any distribution. Dell have always been friends with Linux. Also Daniel Fore (elementary lead developer) uses a Lenovo Yoga with a HiDPI screen and they have been making great strides with HiDPI so you'll likely have a good experience with that as well.
Any suggestions for a good laptop that runs well with Elementary?
Currently (2018), any new gen intel laptops have a good support with debian based distros (like Elementary), so it will run flawless.
The AMDs processors will run... well, the new ryzen is having a little trouble with debians distros as you can see here, but not something that would trouble (only if you are a dev or want go into advanced mode).
For video cards, I have a better experience with Nvidia (and it's drivers), but RADEON cards are good too.
Anywhere to purchase one with Elementary installed?
I saw a laptop in the begin of the year, it was an Elementary OS powered laptop with some additional programs installed by default, but if I was you, I would buy something more powerful and plug it with a bootable elementary usb for a clean install.
do I need to buy a Windows machine and dual boot?
No you don't, you can test it with a bootable USB drive without installing it OR you can install it in dual boot OR wipe it and make a full elementary os laptop.
Lenovo laptops work pretty good with Linux. On the other hand, I don’t recommend HP - I have one of these and they claim that there’s Linux support, but they doesn’t support my model.
I recently bought a Gigabyte Sabre 17. Almost everything works with eOS. The nVidia GPU seems to have a windows-detecting routine built-in - Nvidia's own Linux installer can't even find it. But if you are not a gamer or a heavy video editor, Intel Graphics are perfectly fine.
If you are looking for value for money rather than bragging rights, I can recommend Gigabyte.
Dell XPS 13 - Developer Edition - designed with Ubuntu in mind, hence elementary works OOB :)