In reply to The Potato:
There are so many "it depends" to answer first I be tempted to take Billhook's suggestion! Also one off lenses, even cheap shit moulded plastic condenser lenses are probably of a cost that makes it uneconomic to bother.
So the first "it depends" question is: is it a single LED? how big is it? does it already have a lens (very common bubble of clear plastic moulded into the same package? what is the current angular spread? and finally how far from the LED to the front of the torch?
You cheapest source of lenses will probably be a pair of high dioptre reading glasses but the lens is likely to be too big!
Post edited at 19:41