Those starters have a SLA sealed-lead-acid battery in them, usually around 7Ah to 12Ah size.
For field power, places like Altronics and Jaycar have the same SLA batteries(not starter) pretty cheap.
If you go for one of these, never discharge the battery below 11v(10.5v worst case) and be sure to get a decent SLA charger for them, it can make a huge difference to the life of the battery. Charge them often, Lead-acids don't like to sit idle for more than a couple of weeks.
Miss-treated Lead-acid's have a life measured in months, well-treated they can last upwards of 10 years.
Altronics
Jaycar
Compare prices as they do vary.