Interesting - I've never looked for a basic 18" upper, so I was surprised to see there's nothing out there like that.
Just throwing out a couple other barrel options for you to build up or have someone else do the build.
Brownells has 18" FN CHF barrels for $300 (
link) and Criterion 18" rifle barrels for $269 (
link)
RifleGear has 18" FN CHF barrels for $289 (
link). Rifle gear may be able to build the upper for you with a FSB installed and their prices and services are similar to Rainier in my experience.
Otherwise here's a similarly priced option to what you would pay for an 18" FSB upper - use a 18" barrel with a lopro gas block like this one for $180 with gas block (
link), put on a ALG, Midwest Industries, or Troy rail for $150 (Primary Arms has Troy rails for about $120), give or take. Then you will be at the same price because of the $180 barrel with block vs $300 barrel + FSB + gunsmithing. Weight difference will be negligible, as has already been stated. You could also build that yourself, only special tool you would need is a barrel nut wrench (maybe not even that - depending on the rail you select, because the options I mentioned come with a wrench).