King Arthur will come to your rescue!
Barley malt powdered
They do also offer the liquid, as well as rolled barley flakes (a
different thing altogether, but fun in bread)
Totally unrelated to barley, I have been enjoying making sourdough
following Ken Forkish's Flour Water Salt Yeast... and KA has the
"first clear" flour that brings out the tang delightfully. While
shipping dry ingredients can get costly, they've recently established
a riff on Amazon Prime, where you get free shipping for a reasonable
annual membership fee ... and they send you a little kitchen gift for
your birthday!
As for Forkish, his coaching allowed me to establish my own starter
from scratch, which has maintained nicely for a couple of years
now. My only complaint is that he wants me to toss out an unholy
amount of perfectly usable starter, so I've used a lot for very tasty
crackers (Thanks to KA and their now-discontinued Baking Sheet) and
come to realize you can toss the starter into almost any yeast-bread
recipe and come up with something good. I've even picked up a recipe
for sourdough scones, though that's a bit of an acquired taste.
What I've found interesting is that his baking method is very similar
to the "no-knead" idea that went viral a couple years back, EXCEPT
that the dough -- and therefore the FLAVOR -- develops over a couple
of days. I often found the "no-knead" to be passable but bland ...
give those yeasts some running room and you come up with a really
deep, bold flavor.
So, not that anyone asked for this advice, but even if you're just
making a couple loaves of "regular" yeast bread ... if you can take
the time for one additional rise, you may well find that it's the
best bread you've made in a while!