Flat warm roof - which vapour barrier

[Taliz]

Hi
I am replacing my flat felt roof with a fibreglass warm roof. I was initially recommended to use visqueen fully bonded vapour barrier beneath the insulation as it is self adhesive and there is no need to prime the OSB. However, I need 26 meters and it comes in 20m rolls for £198.

What other options do I have where I won't get as much wastage?

Thanks

 

[Taliz]

Or is it possibly to buy this per linear meter any where?

 

[exotic]

Well if you could find another way to bond it, then there is this for a fraction of the price.