There aren't a whole lot of options to pick from for 2.25" vacuum AIs, so unless you find a deal, you are going to pay through the nose. If this would be your only Vac instrument, you are far better going electric with a battery backup, it would be cheaper overall and less maintenance.
MGL makes a 2.25" electric attitude indicator. You would have to add a separate backup battery though. The AI also has a turn coordinator function, so it can serve multiple purposes.
Power consumption is listed as 73mA for the instrument and 120mA for the ADAHRS, so it wouldn't take much to power.
TCW sells a backup battery system. Their small one is 3AH, which would run the MGL until you die from old age. Heck, a regular 9v would run it for a good hour or two.