I should add some of the bits of my email that Chris didn't quote:
The V150H was the first likely battery I picked out of the CPC catalogue - it wasn't an exhaustive search and I imagine there are others suitable.
As nodoubt says any 1.2V NiCd would do - but watch self-discharge. It's better to get one designed as a memory backup battery rather than a consumer NiCd for cameras etc, since consumer NiCds can go flat (self-discharge) in the order of 3 months even when not used. You can still use a consumer NiCd, but you won't be able to leave your machine off for months and keep the settings. NiMH self-discharge is worse than NiCd.
For anyone who wants the gory details, the NiCd is charged off 5V through two 180 ohm resistors in series (one R130 at the positive terminal, the other R212 at the negative). That sets a trickle charge current of 13mA. For a battery of capacity C=280mAh that's C/20 charging. If I were to put in a smaller capacity like the V150H I'd want to keep this at C/20, so would want to restrict the current to 120/20=6mA. That means increasing the total series resistance to 5V/6mA = 833 ohms, or adding another 470 ohm resistor in series with the battery positive terminal.
Personally I'd go for an alkaline 1.5V button cell in a vertical mounting holder with a Schottky diode in the positive leg (diode anode towards the battery) which would roughly fit the mounting position of the NiCd in a RiscPC.