Gane has a wide selection of capping substrates:

 
BP280 White bleached 50 pound absorbent creped kraft capping material.
Application: Otabind.
Put up: 500 yard rolls/54" wide
BP-282 White Hi-Stretch Heavy weight Bleached kraft where more strength is required.
Application: Otabind and glue binding.
Put up: 500 yard rolls/56" wide
AB-15 White bleached 55 pound creped kraft, less absorbent capping material.
Application: Otabind.
Put up: 500 yard rolls/41" wide
A-5000 Standard, Heavy Duty Latex. Industry standard for stretch paper application. Excellent choice for endsheet combining and book block capping.
Application: Otabind, side feed capping and endsheet combining.
Put up: 500 yard rolls/54.5" wide
A-5505 High strength heavy duty latex impregnated paper, 6 mil spine capping and caseline material.
Application: Otabind, side feed capping and endsheet combining.
Put up: 500 yard rolls/39" wide
605 Creped light weight saturated bleached kraft with superior porosity. Excellent strength in the machine direction.
Application: Otabind, Spine capping for thin books.
Put up: 500 yard rolls/47" wide
A-4541-SNS

SNS creped brown kraft, shows excellent elongation in both directions. For added strength in stretch paper.
Application: Otabind.
Put up: 500 yard rolls/48" wide