What gmp says is true -- it's a very common mistake -- but it's still considered a mistake, a sloppy usage, and it's certainly one that's more common among less educated speakers. There's really nothing all that hard about 'Where're' and 'There're,' and those are definitely the correct forms, even casually.
As for the original question, I don't think there's normally any ambiguity about 'Where're my glasses?', because in real life, no one is likely to be searching for more than one pair at a time. The same goes for scissors, pants, gloves, and so on.
In some other context -- say, you had brought six pairs of plastic Groucho glasses to a party, complete with nose and mustache, and couldn't find any of them when you were ready to go home -- you would probably add the word 'all': 'Where're all my glasses?'