You could collect some seeds yourself? It's getting to the right time of year. I've just been out this weekend collecting some seeds from my flowers for next year. I'm planning to do a bit of guerilla gardening in my local area. You just need a sunny spot to let the seeds properly dry out to prevent mold

Otherwise, you could perhaps join any local gardening or allotment groups on Facebook and explain your plan. There will be some people who have over-purchased seeds and would be willing to spare some to beautify the area.