Two of them are available on Amazon as singles:
Ultra Pro 18 x 24 Top Loader (1 Pack)
http://www.amazon.com/Ultra-Pro-Top-.../dp/B002MUKPUG
One (1) Ultra PRO 24" x 36" Top Loader
http://www.amazon.com/One-Ultra-PRO-.../dp/B004L17318
Have a nice day,
Steve