I'm looking for the same. I've added some red clematis.

Nothing quite like bougs, but you could consider KnockOut Roses for continuous summer blooms. Hey, they have thorns too, similar to bougs!

Not really a good imitation of Bougainvillea, but equally vivid in bloom for shear massive color are the tender perennials(zone 9/10),or annuals for you, Thunbergera alata and T. gregori. Both are very fast growing and bloom while small once it gets warm. Mina lobata is also worth looking into, but not as vividly colorful in bloom.

Might I suggest >

It is considered invasive in many places, but it does remind me of bougainvillea. For many years, it was the favorite plant on the western side of porches because it created a very dense green vine to shade the porch swing.

I planted one in a whiskey barrel to contain the roots, and used tall bamboo poles to let it grow tall. It does grow from seeds, but sort of like the four o'clock plants, it makes a sizable tuber or corm, from which it regrows each year. I am not sure of the zone hardiness of this plant, but it takes it a couple of years to bloom. The leaves are heart shaped, crinkled up, and it climbs with curly tendrils not platelets, so it does not damage your home exterior.

If you are growing stuff in zone 5, that is pretty cold. Have you thought about the huge climbing hydrangeas? I've seen those plants higher than a house in Boston. Of course not pink blossoms such as bougainvillea, but impressive none the less.

I think Climbing roses look a lot like bougainvilleas, or at least give a similar effect. They are thorny and they are colorful, just like bougianvilleas. Thats about the closest you can get to a bougainvillea in a temperate climate, but they do great in pots too!
I have one in a large pot that was a small 6 inch tall plant back in 2009. Today its taller than me and its one of the major focal points of my yard. It cant be ignored because its flower colors are so bright and it just fills up the entire plant! Full sun and little water will get it blooming well and if your looking for size, then water and fertilizer!
-Alex

Not down to Zone 5, but down to about 6b, Pyracantha is reminiscent of Bougainvilleas? Not only the vivid colors remind me of Bougainvilleas--also those thorns. They are also evergreen and those berries are around all winter. Similarly down to 6b, Crape myrtle while not a vine, can be espaliered and those flowers really replicate the purples, pinks, and reds of Bourgainvillea. Maybe someone from lower zones can think of something.

Here is a link that might be useful:

You might like hyacinth bean vine. Easy to grow from seed each spring.

Bignonia is an attractive vine. Some people confuse it with Campsis, because they are in the same family (Bignonaceae) and look similar. But the Campsis can be very invasive and grows a very strong and potentially destructive root system, whereas Bignonia is much less of a problem. It's still a good idea to think about where you are planting it. But the trumpet shaped flowers are attractive, and it grows into a large vine. Just need to be sure which one you are planting, and use common sense to avoid problem locations.