I know a specialist framing shop can make custom frame sizes but are there any places that make this frame size as a pre-set item, for a panorama?
Professional framing suppliers like Pictureframes.com will sell you the components for whatever size you want. Metal frames are usually sold in pairs of sides, so for a 4x18" frame, you would order a 4" pair and an 18" pair. The sides are then easily assembled with hardware they provide. They will also cut glazing, backing, and matting to whatever size you need.
You can get a frame intended for 3 landscape 4x6" photos and then get a bespoke mount.
I saw some Nielsen frames in UK shop called "Athena". They also cut the mounts for £5.
Does anyone have any better suggestions?
If you google 4X18 frame, several results come up. Most are for the same brand name, Craig, and are fulfilled by Sears/K-mart.
If you want a frame large enough for a matte around your 4x18 photo, then you would of course need to find a larger frame. A two inch matte would require an 8x22 frame. A google search for "8x22 frame" yields results from the same sources as the 4x18 search.