Please try to keep the posts helpfull....
Hey Fros, if you have an auto parts store near you, try to go in there on a Tuesday-Thursday around 10:00 am, they aren't busy then, and you'll likely find a person who's willing to help you pick out a heatercore that's the size you need. Just take the measurements you need with you.
You might have to modify the inlet/outlet of the heatercore to match the size of tubing you choose (most commonly 3/8" or 1/2"). There's literally dozens of ways to do this easily though.
The pumps are easier to find online. I've been to dozens of stores in my area, and they don't carry anything usefull.
The tubing can be bought online also, but if budget is a concern, you can use the pvc tubing from a local hardware store. Just make sure it has a thick enough wall so that the tubing doesn't colapse when you bend it much.
I've used vinyl and pvc tubing before myself, and just bought some today. Sure it's not clearflex, but it carries water just the same.
If you have anymore questions, you've come to the right place.