Are there any Fiber Supplement/Laxative which are kosher for pesach
Yes there are. Although Benefiber powder is chometz and can not be used, you may use
Matamucil Original Coarse (powder)
Phillips Milk of Magnesia (caplets), or Phillips Milk of Magnesia ORIGINAL (not feresh mint) Liquid.
If you want more suggestions, I would recommend you look in R’ Blumenkrantz’s book, or look on Pg. 67 of the CRC’s Pesach Guide.