if your shop's legit they can order them for you....
most shops don't have spare lenses in stock.
to give you an idea, i'm waiting for a HI yellow lense for my oakley splices and they're gonna cost around 70$
but on the other hand, polarized fire iridium lenses can go up to 160$
(got this from the oakley website)
i have no idea what spy lenses will cost, probably a lot less then oakley, but i just mentioned this to give you an idea....
(look on the spy website to decide what lense shade you want, get the correct serial number for YOUR model !! and go to your shop and have them order it ....)