Check the front fork of the cruiser, you may find that there is already a mounting hole for a caliper brake in there. If so you can just mount a brake on giving the cruiser the coaster brake and a front brake. The rear will likely not have a mount hole that is usable, or such has been my experience thus far.

I've found innertubes to be pretty forgiving in that if you've got near to the right size you should be ok. Of course a 26" tube won't work on a 27" wheel and vice versa/etc.

One way to source parts is to try putting a 'wanted' ad on your local freecycle www.freecycle.org ask for junk bikes for parts and hopefully you'll get some. People were very generous with me when I did that and I got a lot of bkes to learn on. It's nice to have some extra wheels to learn how to true them and other stuff to work on that you won't care if you make a mistake. This is what the old dept. store bikes are best for. Who knows, you might get lucky and turn up an old 3-speed hub w/coaster brake.

If you don't turn up a brake caliper and lever, then shoot me a pm, I've probably got one I can send you for the price of shipping. I know I've got extra calipers, it's the lever I'm unsure about right now.