There are many ways you can get seeds for free or little money.

Fraternités Ouvrières Seed Shop

I bought most of my seeds at the Fraternités Ouvrières seed shop, in Mouscron. This permaculture adepts have a wonderful garden and a non-profit seed shop (basically a consumers group buying seeds in large quantities for the community). For 30€, I could get 40 different kind of seeds of many diverse varieties.

Be careful, some seeds may be old and not viable any more.

Graines de Troc

Graines de troc is a very nice website, where you can barter you seeds with other gardeners. When you propose new varieties to the members, you get some free tokens to get seeds.

Plant Catching

Plant Catching is a website based on the gift economy. No barter here, you just propose your extra seeds and plants, and you can get the ones from the other people in your neighborhood.

Pro Specie Rara

Pro Specie Rara is a Geneva-based NGO. I got free tomato seeds from one of their conservation operation

From Friends

Rasa gave me some of her seeds, and some seed exchanges are planned with other friends in Brussels, Paris and Switzerland.