You can build a defined space for a garden using fences and terrain paints, but like the others said, it's not necessary.
The best places to look for seeds is in parks and sometimes by the cemetery, but really you can just buy produce from the grocery store or harvest it wild and plant it (if your gardening level is high enough). Another option to get started is to read a gardening book to get your first level. You get about 6-8 common seeds when you learn the skill for the first time.