I have bought from:
MSQC
Connecting Threads
Craftsy
Quilt Shows
LQS, one does not have a big variety of precuts, another cuts their own from Batiks as they are finding it hard to source them for their customers.
Quilt Guild Garage Sale too.

Add Fat Quarter Shop to the above lists

I've found both at Tuesday Morning. Supply is limited, but price is good.

Quilt in a Day has great daily deals too.
I buy when the price is 40% or more off the regular price. Usually Fat Quarter Shop Flash Sales, Missouri Star and QIAD daily deals. These three shops are my favorite. Great customer service and between the three they have everything I want and need for quilting. 20% off is not a good sale to me.

Green Fairy is my favorite, they have deals all the time and monthly clubs too

I cut my own . I have a huge stash I am trying to use.

MSQC often has precuts in their daily deals (charms and layer cakes) but I’m lucky that our LQS has a great variety too.

Green Fairy Quilts has a nice selection

Greenfairy quilts usually has good prices on precuts. The shipping is reasonable too.

I love both Connecting Threads and MSQC!