Hi, and welcome to MINI2.
This came up a few months ago, and unfortunately I think the general consensus was that it is not possible to buy just the lens.
You might have some luck on e-bay, as often a pair of OEM lamps go for far less than the dealer prices.
I also think that the Wipac lamps are the same design as the OEM's, someone like mikeythemini on here will be able to confirm that for you.
If this is the case then they will be much cheaper than going to MINI for the parts.
James.