I save most of my seeds myself, making sure I have a good genetic base for saving these. It’s in fact quite easy, just be aware of possible cross-pollination., and always take the finest-looking healhiest plants for seed-production.  I won’t go into much details here, there are good books on the market, and some very interesting sites can be found.  Here are some major lines:

First of all, many things will cross, if you grow out two pepper varieties in a small space, chances are great they will cross, the same rule counts for nearly all vegetables, tomatoes included.  Rumours are tomatoes will not cross, but they do produce a small percentage of  crosses. Of course, you can grow out one variety of peppers or tomato or beans,or,… and then you have nothing to worry about, with the exception of a nearby neighbour garden with similar varieties.

Sometimes only one precious seed of an old grandmother has formed a plant.  Can one take seeds from this plant?  Well, it depends a bit on the vegetable it is, carrots, onions ( these seeds have a short viability anyway), cabbages and some others are outbreeding plants, meaning they need a cross-pollinisator , otherwise they will not produce seeds, a minimum of 15-20 plants of these is required (for carrots even more, corn requires a minimum of 250 plants) ) to produce good  seeds.  So you won’t be able to produce good seeds from this one plant.  Some othe r vegetables however will produce viable seeds from just one plant, although it is really not recommended.  But if it is the only pepper, tomato  squash,… plant from a precious variety, you can harvest ripe seeds from it,make sure the seeds can ripen off and store them in a dry place with as little temperature changes as possible.  You can use paper bags to store the seeds, or plastic seed bags , these plastic bags are durable but be aware that you have to put in very very dry seeds, otherwise seeds won’t survive for a long time.

Here’s something more detailed on growing cabbage seeds: