There are many weeds called sorrel. You may want to seek plants by their botanical names, or at least link to a pic of what you want for assured accuracy. Curious about your desire to grow this, as I'm sure following readers will be... I'll be the first to ask - why?

There are a large number of plants known as "Sorrel" and many known as "wood Sorrel". Many people consider Yellow Wood Sorrel an noxious "weed" and do everything possible to eliminate it. Some sorrels, French for example, are grown as food crops while others, Sheep sorrels, are eradicted as noxious weeds.
So which Wood Sorrel are you seeking?

The wood sorrel I'm looking for is oxalis stricta.

Yellow Wood Sorrel, which grows so prolifically around here I have never given a thought to saving seeds. The birds do a good job of delivering seed that I don't need to think about saving any. Most everyone I know considers this a "weed" and yanks them out as soon as they are seen, most often before they produce seed.
There may be some of the Wild Flower seed sellers that have seeds for this.

Yes, it is an unpopular but tasty plant. As a boy, we drove this plant extinct on our country land because we six kids loved the sour taste.

There was another plant we called sorrel, but a very different plant, with a tall stalk, which we also loved. Very sour. Either red sorrel or sheep sorrel. Good times. Used to trespass to get more.

mtgs, theoretically it should be easy to collect seeds at the right time from neighborhood, parks, country roads, that kind of thing. But I know from experience that when one goes seed-hunting for some wild thing, the seeds are already gone or it's usually the one plant you can't find although you noted its' abundance in the area before. But, of course, being a weed, it's sneaky and has disguised/camouflaged itself, right? Not seen any around this area, sorry I can't help. Good luck!

Interesting how our culture has let chemical/grass/seed/lawn companies, farmers, golf courses, and even some gardening groups tell us some native food plants are weeds, and believed it. There are some good books on this subject. Anyone interested is welcome to email me.