Inverters costs vary depending on make, capacity and source. Because inverters are not very efficient, it takes a lot of battery power to operate an inverter. This must be considered when deciding what size batter to buy. They are found in catalogs, discount stores, electrical wholesalers and electronic stores. The ampere-hours rating of the batter must be considered as well to ensure sufficient power for the anticipated outage.
Automotive batteries are the least expensive and are readily available.
Most are now sealed so danger from sparks is minimal. However, caution should be used when handling and hooking up automotive batteries. They are available from service stations and discount auto stores. Gel type batteries are available for this service and will probably last longer, but are more expensive. Availability - see your Yellow Pages.