Which master links for 1/8 inch aka BMX chains?
I suggest asking your local bike shop (LBS) because I've never seen a bike shop that doesn't have them, and they're only a couple of bucks. I found the three piece master links easily enough (on eBay and Amazon). You might prefer the two piece style in silver (Amazon)

I searched for "bicycle master link" rather than BMX to get those results.

For single speed chain I'd always use a master link, but with the narrower chains I often don't bother. But I rarely take the chain off the bike, preferring to run it through the chain cleaner on the bike (but I don't get my chains very dirty). For a single-speed it's possible that a really dirty chain would be easier to clean off the bike, but I still try on the bike first because itś so much easier.