As several other have already noted, I always use and recommend either good quality Chinese rooster necks (which have become hard to find because of the avarian flu the last few years) or Whiting American Hackle rooster necks. Saddle hackle doesn't cut it for Glasso style spey wings, the stem is too fine, the stem is too limp to hold the wing shape, the barbs are too short for a good wing silouette, and the barbs are too fine.
With Whiting American Hackle ($20.00 - #30.00/neck), you only need 2 hackles for each side regardless how large a fly you tie because they have sufficient stem strength and barb length to tie even #3/0 Alec Jackson Spey Hooks. The neck also provides the proper left and right curvature you need for matched paired hackle wings.
Strung saddle is the absolute worse thing you can use for Glasso style wings because they taper too much at the tip, are inconsistent in the taper from hackle to hackle, you need to go through a lot of hackle to find matched lefts and rights, etc.
Schlappen (which is what Jack's spey hackle is) is a wonderful spey feather (i.e. body hackle for spey and dee flies); however, it is a waste of good spey feathers to use them for Glasso style wings. If you use them, you have the problem of going through a lot of feathers to find matching lefts and rights, many of the stem are too thin to support a Glasso wing, many of them taper to a far too fine tip for a good Glasso wing, etc.
Spend the few extra dollars and get the Whiting American rooster necks. Trust me, you will be very glad you did, and one neck will tie more flies than most tyers will tie is many years. I get about 25 dozen Glasso style wings out of a singe Whiting American rooster neck. I doubt if most tyers tie 25 dozen spey flies in 5 or 6 years.