The OEM AGM battery is really just a Panasonic and I believe mine just died today which I assume may even be the original battery.
Panasonic Part#S65D26L - A quick look on eBay shows them going for over $600 CAD without tax and shipping! (Lexus online parts sells for about $342 USD).
Toyota/Lexus Part# 28800-31330

Note that both of the above are the same identical part.

Energizer Part#1226340 (Group Size 24F)

One option that I saw listed in the battery parts book was the Energizer 1226340 battery which I paid just $274.99 for. I can confirm it is a good fit and bolts in nicely. Just remember to remove the temp sensor from your original battery and reinstall to the new battery.

Other Lexus forums report other possible options, but whatever you get make sure you get the correct terminal orientation and that it must be an AGM otherwise you risk fumes and spills in your trunk. I would definitely not experiment with random batteries because they are so crucial and will cause "Hybrid System Issues" and cause your car to just die while driving as I've mentioned, unlike traditional cars that will often just run until you turn off the car.

Symptoms of dying 12V in a GS450h/Lexus Hybrid

There were a few random occasions where I had to try twice to start the car in the previous months. You just question if it's a one off or maybe your foot wasn't on the brake. Fast forward to today after some work on the engine (timing chain job), the car just died randomly while driving and wouldn't restart.

The dash lights said "Check Hybrid System" which sounds scary, but thankfully I was fairly certain it was just the 12V.

When I checked the car I noticed the display screen wouldn't turn on, eventually you get beeping and find almost all the dash lights go off and that the power door locks don't work and you can't even open the trunk.

I put my jump pack on the 12V terminals but it just drained and killed the jump pack.