custom using the likes of this?



http://cgi.ebay.co.uk/1550mAH-Battery-For-...915136003r26115

You can use any Lipo battery in the market, provided that it is rated for 3.7v. Lipo single cell voltage is 3.7v, that means there is only a handful of lipo battery with voltage of 3.7v, 7.4v, 11.1v etc, so don't worry if you get an off-spec battery. 

The mAh is the capacity, the higher the number, the longer it can be put in service and vice-versa. You application probably drain only a few mA, other lipo specification is not important. If you can't find 1550mAh, you can use any rating, it won't affect your device.

So, you can just go to any handphone or camera shop, and show them the battery and say you want the battery which has the exact same size. 

One last thing, all single cell lipo after fully charge is rated at 4.2v, therefore you may find certain lipo battery rated at 4.2v, instead of 3.7v. Don't worry, you can use it. Your device would automatically cut-off the charging when it reach the appropriate voltage.

Last I checked RS online got sell several Li-Ion battery. Not sure about Li-Po.

Or try those Radio Control hobbyist shops. Like this ->RC Battery. Generally RC use larger size battery 7.2v and above, but this shop seems to have a couple of 3.7v choices albeit with smaller current capacity.

Actually I'd advise you to only replace with similar size battery (both voltage and current capacity), and to only get them from reputable sources. I'm somewhat concerned about replacing the battery pack with a smaller or larger unit, as I don't know how your battery charger circuitry works.