Watch Doug's Chip Chat auctions and the special auctions he holds from time to time. I believe one of these chips could have be found there in the past year or so. Possibly, the NCV auction last year?

The 5 is only medium scarce, but usually sells high because it is unusual and attractive. The 25 is much harder to find. When Riviera closes and empties their old vault, these chips could become much more available. No personal knowledge, but that's the way things work. Examples are Dunes, Sands and Stardust.