I recently saw this on a fly tying forum. The tyer saw a pattern in a fly bin, got home and recreated it by memory and then asked other forum members if they knew the name of the pattern.
If you want to recreate that fly pattern you saw in whatever kind of shop you saw it in, buy 1-3 flies to have as samples and/or to deconstruct a fly to see how the pattern was tied. Shops will usually have the condiment cups or zip lock baggies to put flies in. Jot down the pattern name, size and where you bought it from for future reference. When you get home, take one out and photograph it for additional reference.