I'm pretty sure Track of the Wolf is the only source for the Lee .480" six-cavity gang moulds, but I checked a little while ago, and they are currently in stock. You will need to get a set of Lee handles for them, and the handles are unfortunately out of stock, but I suppose you could "borrow" some handles from another Lee mould until the extras become available.
I think Lee only makes the .480" mould in the six-cavity version. I've never tried a gang mould like that, but based on @B P Maniac Shooter 's recommendation, I may just have to get one. I got a good deal on multiple boxes of Hornady swaged .480" balls a while back, so haven't needed to cast any of that size.
You are correct. If you can find one these days...you can crank out 100's of round balls in no time using a 6 cavity mould. I've about worn mine out making over 10,000 round balls the past 4-5 years. For those of you that prefers those really tight .490's, Lees also makes them in 6 cavity: