Maybe the attached list will help. All are native to the U.S., so well adapted to our environment and wildlife (which they help support). It doesn't have noticeable flowers, but I'm quite fond of the river birch 'Heritage' - lovely exfoliating bark and it grows rather quickly. You'll often see these sold as multi-stemmed trees, which are beautiful, but they really do best and will last longest as a single trunk tree ... Multiple trunks are likely to be a problem down the road.

Amelanchier is another favorite, bearing small white flowers early in the season. Yellowwood has wisteria-like blooms, medium growth rate.

Take care when choosing a fast-growing tree - many tend to have weak limbs that are prone to breakage in storms.

Some of the listed trees may not be easy to find, but are well worth the effort. Trees are planted for the long haul - to mature decades hence, so choose well and be sure to prepare the site and plant it properly. You'll be glad you did.

Here is a link that might be useful: 9 Trees for Zone 5

If you order plants that are delivered by mail or other del. service, while it may be cheaper and the variety to choose from may be greater, the plants will be much smaller per dollar spent. However, if one is after something specific, sometimes this is the only way to get it. But for the most part, you'll get a better value if you can find it local. It may be that the reason you're not finding what you want locally is because the selection at this time of the year is minimal. Spring is when all the nurseries stock up.

If you supply good growing conditions (supplemental water) during the first two years, it's likely that whatever tree you choose would grow quickly. It's a maintenance duty to remove lower branches as the tree grows and this helps direct the growth to a higher level. So I wouldn't worry about needing to stoop "forever" under the canopy.

I would consider placing less emphasis on trees that provide showy "flowers" as it diminishes some of the better choices. The form and overall habits seem much more important. A row of patented red maples -- like 'October Glory' -- would be nice. While they flower, I don't think it's as showy as you're looking for. However, the fall color is spectacular and would more than make up for it.

For a lawn tree that is showy flowering, redbud would be among the better choices. It's capable of growing quickly and getting large enough that it can be walked under. Another one along those lines is Japanese tree lilac (Syringa reticulata) ... but probably more difficult to find. Regular saucer magnolia (M. x soulangiana) would get large enough. Also, consider mountain ash ... some of the berry clusters are fine looking and long lasting. (Some of the flowering trees (crabs and cherries for example) are produced with a low branching structure. You may need to correct and control this in order to be satisfied with a tree in the lawn.