Glue
EeZeBilts were traditionally made with Balsa Cement - it sets fast and makes the build really rapid. Modern glue technology has superceded this, and so you may not find Balsa Cement easily. If you can't get a large tube of the stuff locally, here is a supplier I used -
Creative Models.
Batteries
It's surprising how hard it is to get a flat battery holder in AA and AAA sizes.
Strikealite do a good selection, with low postage, and they also do very cheap NiMH batteries to go inside...
Motors
There are several cheap motor suppliers around.
Mindset do very cheap prices, though they have a minimum order size. Still, there are so many other strange modelling things on their site that I am sure you will find lots to make up the price....
Balsa Providers.
Balsa seems to be of worse quality nowadays than when I was a lad. Mind you, the sun is also not so good, and as for the beer...
It can be useful to have 1 meter lengths of 4" balsa - this can let you get a whole small boat out of one sheet. I found that
Hobby Power provided me with this size, and a fast and cheap service.
Speed Controllers
We don't need very high amperage controllers, but we do need small cheap ones.
Ebay shops now do ESCs which fit the bill perfectly - I bought a handful at about £2 each...
Radio Gear
If you've got a cheap little boat, do you need a cheap transmitter to go with it?
Hobby King do 2.4 Ghz systems very cheaply, and 2.4 Ghz is very good at interference rejection. We need this, because we're using little engines which may spark a lot, are hard to suppress and we can't move the receiver aerial more than about a foot away....