Re: Price's LMB flies
There's multiple different kinds of "poly rope" what you're after is polypropylene, rather than polyester. You also might be able to find it sold as "macrame yarn".
The main difference between schlappen and saddle hackle is the length of the fibers, and the fact that they're webby. (barbules run the length of the fiber and make them want to stick together, rather than stand out more like dry fly hackles. Webby hackles move differently in water.) Most every pack of saddle hackle I buy has a few larger feathers that are more webby up the stem. Using those in place of schlappen should work fine, provided that the fibers are long enough. (smaller size hooks)