So far as I know, Apple doesn't have any kind of a replacement program even if you have the software coupons, so from a moral standpoint, yes, you could go "extralegal" to find a backup copy of the DVD you lost. Your license is for the software, not the physical disc it came on.
Were I you, though, I'd try to track down a friend with a copy and borrow his/her disc, as you're a lot less likely to download something nasty that way.