That's quite specific
. Whether your motive is for a peaty whisky at that age or if it's more specifically for an Islay whisky at that age, there are just a few options.
Caol Ila release an 18yo at a similar price point and is peated.
Bunnahabhain also bottle an 18yo, slightly cheaper but unpeated and at 46.3%.
Bowmore bottle an 18yo at a similar price point to Laph 18 and is also peated. The old Bowmore 17 can still be found on shelves (it was replaced by the 18yo).
Then there is Bruichladdich. Their core range is a mess and i've no idea if they currently bottling an 18yo. They probably are bottling one or more, plus allsorts around that age. It will be unpeated and possibly wine cask finished.
Lagavulin only bottle a 12, 16, and 21yo. The 16 is readily available. The other standard bottling they do is the Distillers Edition which are also 16yo. All peated.
Due to a period of mothballing Ardbeg doesn't have an 18yo right now, but a few years ago they did bottle the Airigh Nam Beist
in 2006 through to 2008. The vintage was 1990 so they are either 16, 17, or 18yo. Good luck finding one though, but they are out there at the right price (and a favourite whisky of mine).
The last distiller -- Kilchoman will probably be releasing an 18yo as soon as they can, in about 12 years time.
Another Island distiller that produces good peated whisky, and not a million miles from Islay is Talisker (on Skye). Talisker bottle a fine 18yo.
I hope this helps.