Vegetable seeds are different from flax seeds - you always get one back when you grow a vegetable (ie. the seeds are never lost) but reproducing the seeds is hard (or currently unknown how to in some cases) - see the fruit and vegetables page for more details.
It's normally possible to trade for seeds - see the Trading pages (particularly The Goods) for more details.
If you have a paid account, each different University of Worship you visit will give you 4 of a random vegetable seed. There are 11 Universites of Worship in Egypt, so you can get up to 44 seeds (hopefully a fair mix). The following map shows the locations of the UWorships with red lines indicating chariot routes.