Thanks for posting this follow up. It saved me a lot of time. A little more information to help the next person:
The 150A fuse can be ordered from either a Lexus dealership or a Toyota dealership(it doesn't seem like something that is kept in stock). It's part #9098208272.
Once you take the black cover off the fuse panel under the hood you will have to remove the bolt beneath the red "+" clip, unclip the yellow colored fuse block from the adjacent fuse block and pull it out some before you can unscrew the 150a fuse. There is a screw on either side of the fuse block holding the 150a fuse in place. A local auto parts store will likely have a 120A fuse on the shelf that can be used temporarily while you wait for your 150a to arrive. The 120a may need a little modification as the slots the bolts pass through may not be big enough. They can be snipped or ground to a larger size. The 7.5a alt sensing fuse and the 20a radio fuse SirSam mentions is also in the panel that contains the 150a. Read the underside of the panel cover for their location. You may also find some replacement fuses there that have been thoughtfully provided by Lexus.
