looking for driver for canoscan D646U ex , Windows 7 download

Unfortunately that model is not supported on Windows 7 as it's too old so Canon decided not to have any new drivers written for it (having new drivers written costs money so the manufacturer has to make decisions like that to keep costs down). All hardware manufacturer's do that as old models are replaced by new ones.

In other words, a Windows 7 driver for that model does not exist. You can't even try a Windows Vista driver because that doesn't exist either.

It's obviously a very old model because the last driver update was for Windows ME which was launched 15 years ago!

You'll need to buy a new scanner for Windows 7.