I don't know whether this is genuine kit, but IIRC there is a scene in the film
Wings of the Apache (
http://www.imdb.com/title/tt0099575/) where a character takes a Stinger from a downed helo, and there is a kit in a compartment to convert it for shoulder-mounted use. Given that the Stinger is a MANPAD converted for Air-to-Air use this would seem possible.
Even if the ready-to-convert kit is not a real item, a downed helo might provide missiles that could be used with a launcher the PCs already have.
Admittedly a Stinger is not as cool as a Sidewinder, but the Sidewinder is over 3 metres long and weighs 85kg (according to Wikipedia, at least). Perhaps a Harrier cache could contain supplies of Argon to help recharge the seeker on a Stinger?