Hey man! Nkon.nl its pretty nice for getting battery making supplies and cells. Ive never have a problem with tem.
There are not actuelly nickel plates, rather very wide nickel strips. You should be fine with 30mm wide, but if you want more coverage go for 50mm.
I also advice you to go with 0.2mm thickness so you can get high amps before getting the nickel hot, although that has to do also with the ammount of series connections.
Anyways, building a battery, specially for Esk8 its no joke, its drangerous, can have very serious consecuences (like burning your house) and you have to be very certain of what you`re doing. Take in account that the battery will take a lot of abuse from vibrations, so it has to be specially well built.
You should find all the information you need in this thread:
The battery builders club - How-Tos & Esk8 mechanics - esk8.news forums
I know this is long but it is filled with fails and important information you need to succeed!
I did this small condensed safety points that sumes up some of the things you should take in account when building your battery:
Have a look at some posts after that add some information i missed!