I asked at Binny's a few months ago and was told that Bitter Lemon is not available in the US.
Glad to hear that the 1.5L may be available a few places around town. I don't drink it that fast, though, so it would be flat before I finished it.
I ordered the small bottles online from one of those precious British Goods importers last year, but they no longer carry it. I came up with another online source but the price was prohibitive.
In the meantime, I've accepted Fever Tree's Bitter Lemon as a substitute. It's not as good, but it's good. It has that nice bitterness that most lemon sodas lack. It's available at Binny's on North Ave (though they were temporarily out last time I was there, which was three weeks ago).
Speaking of which, I've had Schweppes Bitter Lemon a few times overseas this year, and the stuff I've had outside of the UK (specifically, Australia and Switzerland) didn't taste the same as the original UK variety. I read the ingredients, and I noticed that it didn't have saccharine. Maybe that was it. Lemony but not as bitter.
Here's how I'd rate the different bitter lemons that I've tried:
1. UK Schweppes Bitter Lemon
2. Britvic Bitter Lemon
3. Non-UK Schweppes Bitter Lemon
4. Fever Tree Bitter Lemon
I understand Canada Dry also has a bitter lemon, but I've never seen it in the stores.
BTW, if you fly Delta, they always have a nice supply of SBL in the club at Heathrow. I always take a couple for the plane ride.
J