Hi Gabriel,

I've spent the past couple of minutes wondering why your name rings a bell. I realise now that I bought a small linear actuator off you waaaay back in 2005! Okay, randomness aside...

A couple of good online stores for battery packs are:

Component Shop
http://www.component-shop.co.uk/

And Vapextech
http://www.vapextech.co.uk/acatalog/Hig ... eries.html

They're both suppliers of sub-c sized Nickel Metal Hydride packs, from 4.8V packs up to 12V packs (just connect a couple of packs up in series to get your desired 18V+).
These days, lithium-based batteries are quite popular, especially among featherweights, as they are capable of providing much-improved power output for their size and weight. But for getting back into the sport, I'd say Nimhs would be the best option. Have a look around some of the threads though and there will be some info on lithium packs, rules and suppliers.

Have you got a particular design in mind yet?