Linda, if they are just for you and not for sale or for trades with "braggin' rights", tomatoes' self-pollinating nature ought to get you at LEAST 85% and probably 90-95% self-pollination.
And if you wanted to, you could drape netting over the whole patch so that only vibration, not insects, did ALL the pollination. I think that cross pollination would drop below 1% at that point Or bag some blooms and mark the fruit. 100% pure PLUS nerd bragging rights.
Even without a science project, out of ten plants, 9 or 10 ought to be self-pollinated. And it's not like any one seed can be 80% pure: it's either 100% parent or 50% parent.
The only downside ought to be that a few plants out of however many will be "off". That IS wasted garden space, but if you have enough room, you COULD save your own seeds and just pull a few plants after you see what the fruits look like.
>> I can buy fresh seed each year for a low price.
That's probably the smartest, most practical approach. And then you can trade left-over seeds or seedlings or give them away.
