Simthread from Amazon
Simthread sells polyester sets of thread on Amazon. You get a decent quality of thread at a good price with pretty colors.
I’ve used Simthread purchased from Amazon and found it works well even in more finicky machines. There are higher quality threads on the market, but for getting started with lots of colors that won’t shred and break all the time, I would start with a set of Simthread and build out your collection from there.
NOTE: there may not be a color conversion chart so you can match Simthread color to your design color charts. If matching colors to look JUST like the designer’s photos is important to you, you might consider purchasing a different brand of thread. I like to choose my own colors when I embroider so this is no big deal for me.