Okay, so I'm looking to build a new warbow in the next couple weeks and I need backing material.  I'd like to use bamboo or hickory, but the only people who seem to sell backing slats (3Rivers Archery) only has them in lengths of 72".  The bow I'm going to build is going to be around 78".  So my question is, where does one go about getting backing in above average lengths?

You shouldn't need any backing for a 78in bow! That'd have something like a nice smooth 32in draw. 

Make it a self bow and you'll be fine. You can even use lumber. I've seen wood veneers used as backing though it won't be traditional anymore. I've never tried it but I think Bikewer idea is sound too.

MY suggestion, go to paleoplanet and buy a 78in yew stave. Make a traditonal english style war bow with horn knocks, and you won't be dissapointed.

No joy, Jaap was not there.  Contact Yumi Bows and see if he can help you.  Shipping is going to be a problem, 72 inches is usually the top length, after that you have to pay a surcharge.

If you have a 78in bow you'd probably be okay with a stiff handle. 

Again, I'd really suggest making a self bow. I've never seen a bad tree of birch or maple unless you are talking boards in which case I have no clue. I'd also suggest a harder wood than maple for war bows. Hickory, yew, black loucst, and Osage are your best bets IMO.

If I were you and I (you) am making a 78in war bow I'd buy a 78in Yew stave from paleoplanet trade forums and back it with rawhide and add horn nocks. the total will be less than 80 bucks and you'd have a bow that if probably made will last a lifetime of shooting heavily armored enemies. You should probably be able to make it in the post 100s range in poundage and not have too much of a problem drawing it because the stack will be pretty much non exsistant.

Bamboo is hard to beat TC. That's what I would use.

http://www.bambooandrattan.com/catalog/bamboo.htm

I have a piece that someone gave me with a piece of ipe.  I haven't gotten around to cutting out the bow and gluing it up.  I don't know if this is the way to do it or just his way but here goes.  It is a strip 64 inches long, and 2 inches wide.  The outside is  untouched, he has planed the inside perfectly flat.  The thickness of the strip is 1/8 at the center of the strip, the edges are razor sharp.  This was probably a huge piece of bamboo, at least 4 or 5 inches across at a minimum.  He said to draw out my bow on the ipe, bandsaw it out, clean it up then trace the ipe on the bamboo.  Then bandsaw the bamboo, it will be slightly on the large size because you cut outside the line.  Degrease the ipe and the bamboo with alcohol, score both with a hacksaw blade, degrease again.  Glue up using Titebond and lots of clamps or a bicycle innertube.  Put it in your car, park your car in the sun and roll up the windows and go away for 8 hours.  Take it out and let it sit for 24 hours.  Clean it up, remove the excess glue, sand the edges flush, smooth out the handle, cut the arrow pass if you plan to use one.  Tiller, do it outside as a lot of people have a reaction to the dust from ipe.  That's how he explained it to me, he has some nice, flat shooting longbows, so it works for him.

I have chased a ring on the last five bows that I made.  Osage grows all around me, it's not hard to get some.  What kind of poundage are you shooting for?  And at what draw length?

Bah, the stack will be so low on a 78in bow you should go for 100 lb. Make it 1/4in wider and a little thicker and you'll have a near super bow, also be sure to make the tips really narrow. A 78in bows can be actually be made slower than a 72 in bow so tillering has to be spot on and tips have to be very narrow.