The manifolds join the downpipes (of the flexi' pipe) with copper gaskets. You canna get these from anywhere. even AR dealers can't get them any more...
Your best bet is to try somewhere like Quick Fit .. they're quite generic, I'd have thought.. or try a custom exhaust fitting place.
If you have your old ones, you can re-use them. Heat them up till they're red hot (pliers over the gas cooker may be a bit hairy.. use a blow lamp and a vice if you can) and then quench them in a bucket of freezing cold water.. job done. Use proper hand and eye protection..
I can't remember how the other end of the flexi' pipe joins the cat' .. but last time I was stuck underneath the beast I seem to recall a fibre gasket..? That's different to the Bosal parts breakdown, which shows 2 round (maybe metal?) gaskets:
155 2.5i -V6 06/92-96
Again.. you might have to bother the local exhaust places if AR can't order them.
The bolts... are all M8 x 40mm.. If you have seized/drilled threads in the manifold end you can drill them through and fit a nut.. you may need M8 x 50mm in that case. All the others use nut/bolt/washer as standard. If you're buying new ones (buy new ones!) go for A2 stainless steel. End of exhaust dismantling hassles forever.
The rubber bungs etc. are on the link.. but they also sell them in Halfords etc so you'll find those everywhere.