Performance Products carries both the base/support plate & Tube for the jack post - best to replace both & best to time your purchase for when they offer 15% - 20% off on their weekly specials (sign-up on-line). I found them lowest for it than Pelican or AA - but only after the 15% off deducted, & they all seem to carry the same/similar kits (ditto for battery tray/support, etc.).
They may also carry the seal new, as well as Mark at Mikey914 sig. here in the vendor section with a buch of other seals he makes repro.