Type "u-bolts" into their search box.
Over the years I've wondered why McMaster-Carr never offered galvanized U-bolts. They have plain, zinc plated and stainless steel, etc.
But now they have galvanized for a reasonable price. Great for antenna work. For example, one small 1/4" thread U-bolt costs 46 cents for zinc, 63 cents for galavanized and $2.06 for stainless. The galvanized will last for many years, while the zinc rusts in less that two. The stainless is good forever, but expensive unless you only need a few.
I just put in an order for a second batch of the galvanized. I was so pleased with the first batch I thought I'd post this info. Some of the guys said that they didn't have a source for galvanized these days.