for large pond liners that I know will be durable against ground and gravel, I usually get the EPDM Firestone pond guard. Search for Just Liners Plus as they are one of the better priced I've seen online for large but not huge orders.
Farm Tek or Growers Supply will have some large pond liners for different purposes.
If you want white liner (though it's not that flexible and only comes in 6' widths) you might contact Green Acres for Durascrim.
If you are talking like really really huge wholesale orders of liners, you may need to contact some of the manufacturers directly but this may not be worth it unless you are really talking about freight quantity with more than 100 foot rolls.