First, make sure the current owner of the discus doesn't feed them anything for 24 hours before you pick them up. Use a large clean cooler, like a large, long picnic cooler you could line it with a large plastic bag, but if the cooler is water tight, I wouldn't bother with the plastic. You could use two coolers, even better, and put 3 fish in one and four in the other. Your water will stay cleaner that way. When you pick the discus up, fill the cooler as close to halfway as you can with their existing tank water. Add a dose of dechlor to help convert ammonia. You could tape a heater pack to the inside of the coolers to help maintain their temp during their trip. I don't know when you're picking them up, but if you need a couple of heat packs I could send you two. I don't think there's anyway to keep them from sloshing around during your journey whether in a cooler or a bucket. Another option would be to get some 3 mil bags from a fish store and triple bag them with some O2 added, but I'll warn you, getting them bagged at 5+ inches and having them not puncture the bags with thrashing their sharp dorsal fins isn't going to be easy.
