The best substitute that I have found for Pearsall's Gossamer is Tire Machine Twist 50 weight. It's basically the same and comes in 171 colors.
They also have Buttonhole twist of excellent quality. Here are some colors matches(very close but not exact) for Pearsall's. The first number is for Pearsall's and the names and second numbers for Tire thread.

3 Champaign 203
4 Honeysuckle 29
6A Mustard 74
6b Burnt Gold 8
10 Putty 25
11 Yellow Green 148


www.silkthings.com/catalog.php?cat=001