[ame="http://www.amazon.com/exec/obidos/tg/detail/-/B002MO9MB0/ref=ord_cart_shr?_encoding=UTF8&m=A17MC6HOH9AVE6&v=glance"]Amazon.com: Pro Optic Budget Auto Extention Tube Set for Canon EOS SLR Cameras (Meike):
Adorama Camera[/ame]
[ame="http://www.amazon.com/Kenko-Auto-Extension-Canon-Mount/dp/B000U8Y88M/ref=sr_1_1?ie=UTF8&m=A17MC6HOH9AVE6&s=photo&qid=1266657304&sr=1-1"]Amazon.com: Kenko DG Auto Extension Tube Set for the Canon EOS AF Mount.:
Adorama Camera[/ame]
were the two sets of tubes i was looking at to play with. you can find the nikon equivalent if you're interested. they have the electrical contacts so you wont lose auto focus or aperture control.
these two are actually the very two derrel speaks about above.
as for your filter question...
Is that true? If so is it one size fits most?
no. you will need to find out what filter size your lens takes. its usually printed on the front of the lens (if you're looking straight at the glass) on a little ring encompassing the diameter of the lens. it'll have several markings:
if you take a look you'll notice in this image the outer ring on the front of the lens has quite a few markings like "Canon Zoom Lens EF 28-135mm 1:3.5-5.6 IS" on the top, and on the bottom "Canon Lens Made In Japan". Off to the bottom left a bit is a little circle with a line through it and 72mm printed next to it. that is telling you that the filter size for that lens is 72mm so you'll need to purchase 72mm's for them to fit. there's adapters to make larger filters fit smaller lens's, but that's outside of this scope.
if your lens doesn't have these markings anywhere, you can google to find out the filter size for your lens or check some place that sells the lens like amazon,
b&h,
adorama, etc. and they'll list the specs of the lens.