Batteries

1/4" thick... pretty small for a battery.

This about 2-1/2" square X 7/8" thick

Dooorman, thanks. I have never seen this type before. Where can I buy it and what is it typically used for?

You haven't answered any of the questions asked...

If the offered battery will work, here is a source. This in the UK. You haven't told us where you are at, either.

There are a few types of flat (1/4" thick or less) and custom package rechargeable and non-rechargeable batteries in various chemistries on the market.

What is the application, expected current draw (peak and average) and expected battery life?

You can use this cell phone battery. This one has 7.15 hour talk time, 14 days standby.

Hooking several together in a series/parallel configuration can give hours/days. Depending on your amp draw/application.

There are many lithium polymer batteries of roughly this general (flat) layout. Kokam makes a range of sizes. Check with model aircraft suppliers. Cell phone batteries are often flat format also.

What about one of the old Polaroid P500 6V lithium batteries

http://www.smallbattery.company.org.uk/sbc_p500.htm

About 3" x 3.7" x 1/8"

Using two would give you the flexibility of 6V and more mA hrs. or 12V

You haven't specified a voltage.