The scope that medvedqc refers to (Bushnell LRTS 3-12x44) is a great scope, I have a few (as well as the 4.5-18 version) and I've used them to shoot out to 1000ish yards. However, I'd suggest something a bit bigger for a 338 Ultra since you'll be able to push out quite a ways farther. I have a .300 Norma Mag being built right now and I'm putting a Bushnell XRS2 4.5-30 on it. It'd suggest any high end 5-25x or higher scope, Leupold, Night Force, Schmidt, etc.
I have a couple Leupold Mk5 5-25s and they are great scopes as well. If you're looking for a great scope that can be found for relatively cheap, I'd suggest hunting around for a Bushnell HDMR2/DMR2 (it's one step below the XRS2) as they can be found for quite a good deal.