Finding the seeds may require a bit of sleuthing, albeit pleasant, on your part.  A few garden centers may carry a limited selection of heirloom seed packets and perhaps a few heirloom pepper or tomato transplants.  A better bet for locating the specific vintage varieties you seek is to order them from a number of catalogs that either offer some heirlooms or that specialize in them.  Check out the seed companies and seed exchanges resources within this site.

One of the major criticisms of open-pollinated vegetables is their increased vulnerability to disease and insect pest damage.  This, however, has more to do with a given variety's adaptation to your particular area than it does to a generally higher pest susceptibility. The key is to find those varieties that do especially well in your area.  Again, you'll need to do some investigating.  Check with your county extension agent, read up on the various vegetable types you'd like to grow, surf the Net and ask the advice of your fellow local gardeners, including growers who sell at local farmers markets, to learn which ones work best for them.

It is true that vintage types can be more susceptible to disease and so it behooves the savvy gardener to be extra vigilant in keeping an eye on his or her plants.  Depending on where your sensibilities lie along the absolutely-no-chemicals <---> organic <---> whatever-is-needed continuum in terms of disease control, you could apply preventive fungicide to those vegetables especially prone to fungal diseases such as powdery mildew.  In most cases, applying the fungicide after you've witnessed the symptoms is usually, but not always, too late.  If the particular disease is not life threatening but merely unsightly and not affecting the vegetables themselves, simply tolerating it may be the best course.

Other times the best treatment for a diseased plant may be its removal.  In some cases, it won't be able to recover and you might as well get it out of your garden.  This way, you'll also prevent the spread of the disease to other plants in your garden.

Another tip: plant several different varieties of the same vegetable type.  That way, if disease or a particular pest strikes one certain variety, the others might very well be resistant and survive.  What you're doing in this case is avoiding a monoculture of just a single variety.  This monoculture growing environment, by the way, is what caused the great potato famine  (actually a potato blight) in Ireland and northern Europe in the 1840's.

In general, practice good, sound growing techniques by rotating crops (this helps cut down on soil borne diseases such as verticilium wilt), erect physical barriers to keep out insects and critters, fertilize and keep the garden clean by removing and destroying (not composting) diseased plant parts.