Where can I get/buy LG G3 screw replacement?

I just bought my LG G3 two months ago. I bought this phone with an AT&T contract, but I live in Spain. The phone was working perfectly until one night I woke up and it wasn't turning on. I took it to a repair store just to check it (they didn't open it completely as for it to lose warranty) and they lost one of the screws. Now they won't apply the warranty until I get the original screw that's missing. Where can I buy original or, at least, a similar screw?