I think this is about number four in the Ra-Ro Classic top ten of FAQ's!
Lasselles or Exeter for GRP replacement roof-linings; but expect to pay probably as much as you did for the car! (or at least significant proportion there-of!) They aint 'cheap'.
Problem is that the headlining is molded fibreboard, over which is a layer of soft open cell foam, over which is a thin felt and then the lining textile.
With time the bonding between the layers starts to deteriorate, excascerbated by the foam starting to break down.
Some poeple press trim rivits through the fibre-board to clip up the droopy bits; bit ugly, but it works, but wrecks rthe fibre-board under neath.
Most effective way to deal with it is to pull the headlining out. You have to unscrew the trim 'furnarture' like the hand grabs, and rear speakers, and the sun-roof surround if you have one, sun-visors, possibly the mirror if you have the one that screws into the roof......... Then you carefully 'jiggle' it out of the back tail-gate.
Once removed, and good idea to wear a breathing mast here, rip all the old textile off the fibre-board, then lightly 'sand' the fibre board so that its not 'shiny' and coated with old glue, and new cloth will stick to it.
Now, you go to your local market and see what materials they have on the dress making stand........ I am lucky, we have Birmingham 'Flea' market just up the road, and an amazing array of muslins, calicoes and silks on offer for making indian sahri's!
{Had to restrain myself; had ideas of following the beduine tent effect of the drooped liner and doing a kind of persian print silk thing!}
But you get the idea, you find some material that is suitable, in my case I used black vynal, but choice is pretty much up to you.
Got a new one to do, going to see if I can get white vynal this time; or maybe a hessian.
You then get a couple of tins of good old fasioned 'Evo-Stick' contact adhesive, and paint it onto the back of your covering cloth. Then you poor more onto the inside face of the fibre-board liner, spread evenly, and then reasonably swiftly, you lay the cloth, glue side down onto the liner, and working from the curtecy lamp holes, smooth it out towards the edges.
BTW you may have a single piece liner, or a two piece; two piece is a bit easier to do!
Make sure you wrap the cloth around the edges at least by a couple of inches, and get it to 'take' there, before trimming off excess cloth.
Leave over night for the glue to set, then wallop it back into the roof, in those immortal words, by the reverse of removal!
Sorted.
Other sugestions include squeegeeing PVA glue into the fabric from underneath to get it to stick back on the liner...... never seen it done successfully myself, always wrinkles, and makes the material hard and shiny. Spray glue has been used in a similar way, but IF it sticks, often doesn't take for long and you get droops coming back.
One that I HAVE used, but only for small wrinkles when they have appeared, or where edges have lifted, like on the sun-roof screen, is 'UHU' glue, worked in around the effcted area, from underneath. Doesn't look TOO bad as long as it is only on a small area.