I love Baker Creek and Seed Savers Exchange.
Regarding the crossing... it depends on the vegetable. Tomatoes pollinate their own flowers by the way they're built, for the most part. Flies and bees and the wind can mix in some pollen from another plant, so the "isolation distance" isn't very much, 50 feet or so should be enough. So, yes, they can cross pollinate, but they don't, very much. Usually.
Corn, however, is wind pollinated, so it will cross very readily with any other corn. To keep pure seed, the isolation distance is measured in miles, and is impractical for most gardeners if there is ANY kind of corn grown in the area. Hand pollination and bagging are necessary.
It's hard to save seed in the cruciferous family. For instance collards, broccoli, Brussels sprouts, cauliflower, cabbage, kohlrabi and kale will all cross with each other, and isolation distance is about a mile, as they're insect pollinated.
So this can get complicated. Or be easy. I've saved my own tomato seed to great effect. Potatoes are even easier, as just save some to plant the next year. The challenge is to keep them from rotting in storage. Mine have sprouted, but I just plant them deeply enough that the sprouts are underground.
There are several books on saving seeds of various vegetables with good descriptions of what you need to do.