I don't know how Maine is, but around here, the various dealerships will often have parts coupons on their web sites. For example, one of the local ones has a 15 percent off parts bought (or ordered) on Saturday and 10 percent on other days.
No parts coupons here, just "service specials" which are actually rip-offs, plus they're closed on Saturdays except for car sales.
No luck in finding one at a salvage yard, so I checked with the Toyota dealer this morning, and they want $235+tax for a replacement non-smart-key battery, or $21.76 for the battery tray that fits the larger smart-key style battery, which they didn't have and said they'd have to order from the west coast. The Toyota part number for the larger battery tray is 74440-47020, and searching for that online I found that the best deal was to be found at a Toyota dealer in Richmond, Virginia (Toyota Online Parts | toyotaonlineparts.com | Toyota Parts Online) which priced it at $12.06 + 7.85 s&h. So, even with the shipping charge, that's less than what the dealer wanted, and it will probably arrive here sooner. Incidentally, the dealer price for the battery tray is $5 more than Toyota's list price, which goes to show what scoundrels they are. I also found an auto parts store 15 miles from home that has a smart key style battery in stock for $163.69, which is more reasonable than the other parts stores that I called. Interestingly, the online store linked above offers the smaller non-smart-key Toyota battery (part # 2880021181) for about the same price, but shipping would have cost almost $34. I'm picking up the battery today, and the tray should arrive Thursday.
Yesterday evening I threw a 2 amp charge into the old battery for 3 hours. The voltage tested at 12.86v directly afterwards, which didn't look so bad, but by this morning it had already dropped to 12.56, though it is still holding at that level 6 hours later. I can imagine, though, that it would probably have dropped to a critical level if it had been reinstalled in the Prius overnight, since there is always a current draw on the system.
I ordered that today from Amazon, which worked out well because I already had some gift card credit there. I also downloaded the Techstream 10.30.029 software zipfile. Should have the Amazon order by the weekend. Thanks again for the suggestion, as that's a lot better than ordering something from China, even though it's probably made in China or Taiwan, just like everything else these days.
Not wanting to wait until Saturday for my extended size battery tray to arrive, and having purchased the larger smart-key style battery, I decided to see what I could do with the existing battery tray temporarily to allow me to install the new battery and use the car. The battery tray is actually made up from two metal pieces, and joined by two spot welds, so I decided to drill through the welds to break the bond, and then separate the two pieces, discarding the piece that is L-shaped to hold the battery end. I first drilled through the center of these weld points with a 1/4 inch drill, and followed up with a 3/8 inch drill bit. That did the trick, and after separating the two pieces I reinstalled the flat tray in the battery well, dropped the new battery in (Exide AGM group 51), fastened down the retaining bar, hooked up the two electrical connectors and the battery vent tube, and put the remaining items back where they belonged. Incidentally, the battery is firmly held in position, so it would appear that ordering the new battery tray wasn't really essential, and if someone else runs into this problem of the larger battery not fitting into the existing tray, this workaround could get them back on the road quickly and save the $20 parts cost for ordering the larger tray. I'm not sure yet if I will put the larger tray in when it arrives, or sell it on eBay to recoup my cost. Anyways, when I pressed the Start button, the Red Triangle appeared on the dash panel, as well as the red "BRAKE" warning, and I couldn't get to Ready mode, so I shut down, fully applied the emergency brake, pressed the Start button again, and then let the emergency brake off. The BRAKE light went out, and the Prius went to Ready mode, started, and I was then able to shift the transmission and move the car. Thank goodness for that! It's strange that the original 12 volt battery had chosen this time to fail, causing me to think there was another explanation having to do with changing the AT fluid, but at 9 years of service it didn't owe me anything. The new Exide battery will probably last about as long, and is guaranteed for free replacement the first 4 years.