I just ordered a Carver bimini for my 18'.
I got a 4 bow, all stainless with sunbrella fabric.
The one I ordered is the 54 inch high by 75 inch wide model. Carver says it will accommodate 73"-78" wide gunnels.
75"-76" is just about perfect for the 18.
The unit I got retails for about $1100 (but I work for a boatyard, and we're a dealer so I get it at cost.
)
With aluminum tubing and only a 3 bow I think retail would be more like $600.
Greatlakesskipper.com also has a ton of biminis, stainless or aluminum.
Don't get anything wider than 79" and shorter than 50". I'm 6' tall and standing at the console, a 54" high bimini is just about perfect.