You can often find 7A fuses at Radio Shack, or sometimes in the glass fuse variety pack sold at auto parts stores and some gas stations. Ask to check the pack or open one to make sure it´s in there before you buy it.
The high beam on these old bikes is typically only 55W, which, by the power law (P=VI) is less than 5A even at idle. You could probably do just fine switching both 7´s to the more common 5´s. Resist the urge to purchase \