Im in the YL area and also in advertising. I would check out the Mong Kok area (Sai Yee st), or online as beermats will probably not be printed locally but a factory in China - I'd look for a company like this one POP-china.com who are used to multi purpose print materials. Your best bet would really be to inquire with a few companies based in HK and they should point you in the right direction. Although beermats are more specialist I've seen plenty of mousepads given out as promo material so I know multiple format printing is available locally -- just needed to enquire as to where. You could even try a few bars that are specialist e.g Dublin Jacks, Kangaroos, HK Brewhouse etc they might be nice enough to give you the information if you have a few pints! Do NOT get suckered into doing it online at an overseas company us, uk, oz as the postage would not be able to justify the cost when you live 15 mins away from china. You'll have to look carefully but Im sure you'll find something.
Lastly, this is rather mean but I could not help thinking 'ooh, classy wedding souvenir' when I saw the title of this thread hehehe