I need to seal some drilled holes in my 150 and I have bulkhead fittings but can't seem to find a plug that I could use to close them off. Where can I get such a thing? Home Depot? or do I need something designed for aquarium use?

Home Depot sells plugs and caps etc but make sure if you plug that your plug does not extend beyond the nut and interfere with its removal, otherwise next time you will need to use the hole you will find yourself with a hacksaw cutting the bulkhead off. If the bulkhead is threaded, that's another story. But a slip plug needs to be glued.
Any plumbing plug at home depot that's made out of PVC will work. It does not need to be "aquarium-specific", just rinse it off and wipe it down and you are good to go. My thoughts about plugging a bulkhead vs. patching the hole are another story though

okay, okay, you've convinced me. I definitely would not want to crack this tank, though with glass this thick it would have to be quite a blow. Count on me to accidentally provide such a blow....