Figure out what kinds of seeds you are interested in growing. Then choose which of these options best suits you.
Step2
Wintersown.org offers a free six-pack of seeds if you send in a self addressed stamped number 10 envelope. Visit wintersown.org, or the link at the end of this article. Winter Sowing is a proven method of growing plants from seed in mini-greenhouse like containers OUTSIDE. It is a huge success and it's much less expensive than buying plants or growing plants indoors with all the equipment that you need.
Step3
Visit online garden forums such as GardenWeb's Seed Exchange. Many people trade for seeds, sometimes seeds for seeds, or items for seeds. They even offer free seeds to new members from time to time. The wintersown GardenWeb forum also sometimes offers free seeds to those who are new to winter sowing. The forums do state that you shouldn't post to ask for free seeds though, but responding to an offer of free seeds is allowed.
Step4
CheapSeeds and Main Street Seed and Supply are online seed stores that offer incredibly low prices for huge quantities of seeds. The seeds are great. I've ordered from both places with much success. This option is especially beneficial to those who want to buy bulk seed. See their links below.
Tips & Warnings
Winter Sowing has been one of the most successful experiences I've ever had with regards to growing flowers, vegetables, and herbs. I strongly suggest you try winter sowing.