shimano uses 130/74BCD, so I would just get a 24T or 26T chainring and be done with it. (I'd recommend sugino, since it's inner ring is shaped the same as shimano)
then if you really really need a lower gear, you can get a deore 11-34 and take the 26T and 30T from that. they would be replacing the 16T and 25T cogs from the 12-25 cassette.
which would give yourself a nice 12-13-14-15-17-21-23-26-30 cassette.
I expect that the top three gears on the 12-25 come as a one-piece unit and cannot be divided. My favorite large-range cassette is one that I put together using a 12-25 10-speed cassette, removing the 16T and putting a 28T on the end which came from an 11-32 9-speed cassette (I believe Deore and LX cassettes are suitable for this because they are separable, but with XT and XTR cassettes the top three rings are again one unit, so they cannot be used for this). I had to play around with a few different spacers between the 25 and 28 until I found a combo that worked. When I want even lower gearing, I remove the 14T and put the 32T on the end (and use an MTB rear derailleur).