@jlutz,
Plenty of good suggestions for the plant varieties; I'll tackle the other end of the question and add to Dutchlady1's suggestion about asking local nurseries - your local grocery stores which have a floral department; contact the manager or the district manager and see if you can get them on board.
Contact the local branch of the American Legion as ask them to post a notice; some of their membership may be able to help.
Also, post your request in your local FreeCycle network; it costs nothing and usually gets results.
Don't forget the Boy Scouts and Girl Scouts; they are always looking to earn their merit badges and helping you with your cause can help the young people to learn about the sacrifices made by the veterans.
I've worked with the homeless Veterans in our shelter back home; a challenging task buy very rewarding - I applaud you.
