Can try local stores such as Cathay Photo, Orient Photo
You can also look at the mass order section in BnS for people ordering in bulk at better prices.
Some other brands you might have to buy from their online sites
I just bought mine recently. Seems like Cathay is out of it (as of last week, new stock might have arrived this week). Orient Photo had a few copies left and I managed to get a Cokin GND +8. They were out of +4 and +2 but I was told that they have new stocks this week. If you want, better faster go and grab them, cause I heard they get snapped up pretty quick!