I bought a 12x loupe from Edmund Optics; it's very high-quality and cost about $50.
I bought it because I was worried that my 5x loupe was not cutting it any more, as my eyes get older.
So now I use the 5x to get in the zone, and double-check with the 12x.

I use it on both 4x5 and 8x10, and it's especially useful with wider lenses.

If you order from Edmund, make sure you do so over the phone; doing it online will cause your order to be shipped to Canada via UPS.
You will incur "brokerage fees" of up to 50% of your order.
Call them, and ask for USPS shipping.