Most places that sell freeze-dried meals also sell freeze-dried vegetables (corn and mixed vegetables are two I remember seeing - maybe at REI?)

I've also simply bought the Knorr spring vegetable or plain vegetable "soup and recipe mix" and added part of a bag to ramen or rice. You can usually find it with the instant soup or the gravy and sauce mixes.

Dried mushrooms are sometimes available in the grocery - though they're often the more exotic varieties: they're good, but very expensive. Tone's (the spice people) used to sell big jugs of dried, sliced button mushrooms - and also mixed vegetables - but I haven't seen them locally for quite a while. (Sam's Club was my source, with the institutional food.)

Finally, I've also had success with plain old dried peas - the ones you find in bags in the stores, usually somewhere near the rice and pastas. I take them in a ziploc bag, along with a short, wide-mouth container. A couple of hours before the meal, I put the amount I'd need for the meal into the container, add water, and let them rehydrate while I walked.

Later: I went to REI and looked - they have lots of veggies, including one "kit" that has 15 or 20 individual pouches, including green beans, peas, and a bunch of other stuff. Look under "side dishes" after going to the food page.