Here are two reviews of ipad keyboards that have received a favorable writeup. Unfortunately they are on the expensive side – unless you can find one on sale.
The Apple Bluetooth keyboard works very well but is pricey [we have two available for use in the library].
The iPad guy uses a Mathias One keyboard, and is impressed with it [but it is NOT battery powered, and requires a USB power source. On the other hand, it can also be used with a laptop/desktop.] On the OTHER other hand, it’s only $29 from MacSales.