3D Filaprint are my go-to guys for filament. Next day delivery, good prices, great range, and outstanding customer service
They also have
Filament Sample Depot for filament samples which is a very handy site. They do samples on the main site, but they have more selection on FSD. I have many, many wonderful samples of different types and colours. Its very useful to buy a sample of something you want to try before committing to an entire roll.
You won't get flexibles to print on the CR-10S without a lot of messing about and modifications. It's been likened to 'pushing string uphill' since the flexible filament just bunches up in the PTFE tube. That's if you can get it in to the tube, which you can't with the stock extruder since it all squidges out of the top. I did the modifications and messing about and ended up with a number of unusable prints before I gave up. Flexibles really need a direct-drive extruder which is what I'm going to put on the Hypercube.
If you're going to get an Ultrabase, make sure its a genuine Anycubic one. There are many nasty clones around which have problems with the coating coming off. It will have 'Anycubic Ultrabase' printed in the front right corner if its genuine. Most of the clones don't have anything.