Yesa battery, lithium iron phosphate-- http://www.yesa.com.hk/
-low-cost seller of lithium iron phosphate batteries. good customer service, long lasting batteries, but you will have solder on your own battery-connector and make your own protective package to keep the battery electronics safe. the batteries are much lighter/smaller than equivalent-range lead acid or ni-cad batteries. Prices are higher than the lowest-price e-bay lithium iron phosphate batteries, but not by a whole lot. I have one of these batteries and I highly recommend them.

http://ebikes.ca -- sells batteries (and many other e-bike items). Includes ni-cad, SLA, NiMh, and lithium.

DeWalt 36V battery on e-bay
http://search.ebay.com/dewalt-36v-ba...dsreturnedZ300
DeWalt's 36V batteries are said to be excellent for e-bikes but only if you are willing to put a lot of do-it-yourself effort in to repackage them for a reasonable riding range, etc. These 36V (actually closer to 33V) packs are a high quality lithium iron phosphate type.