Ebay is your best bet. Just be sure what you're getting is region free, unless you own a region free player.
On a side note, I've seen the Viewmaster reels for AOTC, and the effect wasn't much better than all the 2-D conversions they used to put out in the old days simply to just have a merchandising tie in. (E.T. was the worst offender in that category.) Whether they are derived from the actual 3-D conversion of Episode II is open to debate.
What was really sad was I had to explain what a Viewmaster was to a Toys R Us employee when I couldn't find them in the store.
I've not been able to see what the ROTS reels look like.