I'm afraid I'm no help either, and my stash is at least 90% solids and blenders. I never buy from the sources you mentioned and don't buy pre-cuts for the same reasons as pocoellie.
However, if you'd like suggestions on how to add solids and blenders to a stash in general, without those size or price constrictions, it becomes easier to offer help. Look at what you have for prints and you'll most likely see a tendency towards one or two, three tops, colorways. Those are the colors you gravitate towards instinctively. Look for solids, blenders and tonals that merge well with what you have on hand.
Some people suggest using the dye dots on prints to select coordinating fabrics of any type, soilds included. That works well if color choice is difficult for you, but I've found that a variety of values works far better than matching the dots precisely. The result is more depth and movement in the quilt when all are cut and pieced and combined.