Our local WalMart has them in the jewelry/craft section for $1 for 100 1x2" bags & $2 for the larger 2x3" size! I tried buying the manilla coin envelopes in the past from office supply stores & the small seeds fall out of the edges!
We buy a lettuce mix at Sam's Wholesale Club & I use the empty plastic boxes to collect seed heads & let them finish drying in my garage!
Take care,
Lila