Window glass would probably work for a smaller tank, but not for a 150. Plate glass at 3/16" is about as thick as it gets for windows. I go to ace hardware to get window glass cut for tank lids and it is thinner then your typical glass top to the point where the glass sags on my 3 ft tank. The glass they sell is thinner 1/8" I think. Getting a replacement piece of glass for the 150 you will have to go to a specific store for glass that can do custom orders, but its not likely going to be cheap for a large piece of thick glass. If you want to replace it with non-tempered you need to look into the thickness it should be. Non-tempered will likely have to be thicker then what the tempered sheet was, since tempered glass is much stronger. Then there is the work of removing the bottom and top trim(hopefully not damaging the other panes) then removing the broken pane and putting the new one in. Then the issue if the trim would fit back on if the replacement pane was thicker. Trim is very important for tempered glass as it is actually weak to impacts on the edges then typical glass. Once you break a pane there is no fixing it.
You could try to fix it using plywood but, I have very little knowledge of how this is done.
If the side pane is tempered the entire tank will be. Its a lesson learned the hard way but you should always check for tempered glass whenever drilling a large tank. Above 55 gallons tempered glass is pretty common. My first 55 gallon tank was fully tempered according to the sticker. I replaced it after 7 years when the brace started giving out and the new 55 is non-tempered with noticeably thicker glass. There are plenty of videos on Youtube of how to identify tempered glass, it involves a LCD/LED screen(cellphone usually these are polarized) and another polarized filter(camera or sunglasses). Make sure you check both side and bottom panes. Some tanks can have all panes tempered, some only the bottom pane, and other will not be tempered at all.
