I would suggest that you avoid buying those wildflower seed mixes even if they do come from a reliable source. Why? You're never quite sure what you're getting and you may end up with a bunch of pesky weeds or invasive species. Sure California poppies are cute but in Portland they can be a menace on the scale of dandelions and annoying hipsters clogging up the sidewalks and byways.
It can be fun and easy to just make your own wildflower mix. This way you can be assured that you're getting the plant that you want and the ratio that you want. Buy your favorites and mix them in a cup or don't mix them at all-sow them in groupings if you want a slightly less wild look. Do a little research on each flower you're growing so you don't end up with yard that belongs on the noxious plant website. Then, pray for a little sun this summer.