you are correct. trying to match colors exactly is exhausting and frustrating. that's true even if you're lucky enough to have an in-person place to shop for them.
consider changing your strategy a bit. remember that works of art inspired by other works of art are nearly always "interpretive". that is to say that they don't have to be exactly correct in order to be beautiful and appreciated.
as long as the colors you use create the appropriate look, it won't matter if they aren't super-precisely accurate. cross my heart. ;-)
is there a place near you that sells paint? they might not mind if you collected paint sample strips that show the colors you need. then, contact HOP to see if you can mail them the chips so they can go through their bolts to match those colors for you. if they can't, try contacting Mary Jo's. i don't think they carry moda, but they have a huge selection of tonals from a number of designers. (i'll bet it's the same at HOP.) it's possible that drawing your fabrics from different collections will add interest to the quilt you hadn't expected. :-)
you might also consider buying a supply of fabric paints and some nice plain white muslin. you can mix and paint your own. i could be wrong, but considering the price of the fabs you've been considering, i believe painting your own will save you piles of money in the long run.
when you've made what you need for your project, you can also experiment with faric painting for its own sake. there's a huge market for hand painted fabs. if you find you have a talent for producing beautiful fabs, you will most likely be able to sell them. :-)