(1) Aluminum tubing.
I get my tubing from Ace Hardware. They usually have a small stand (which is usually found in near another parts counter or in a corner) that has all sorts of small brass and aluminum pieces, such as round or square tubings, rods, brass or aluminum sheets, etc. Beveled style stacks are best using aluminum tubing. Once you have your tubing, heres a small reference I use...
1/8" = A 6in. stack
5/32" = A 7in. stack
For beveled stacks, cut the tubing to length and trim up ends with a file (not really necessary). I use a bench grinder to make the beveled ends. The key point to making beveled stacks is that you don't slant it too much and to keep the same length. For the elbows, I'll use a piece of rod that is smaller that in diameter of your stack, either if its a brass rod, nail, wire, etc. as long as it looks good. I stick mine in a vise and hammer it into a 90 degree angle and cut to the right length and then glue into the end of the stack. For finish, I'll use my Dremmel tool with a polishing attachment and use some polishing compound and slowly polish the stack. You can also use chrome tape, bare-metal foil, or chrome bumper spray paint.

For curved stacks using aluminum tubing, you can buy a tubing bender that can be found at the same rack where you found your tubing, other benders can be used also. Kinking is a big problem here. To prevent it, stick a piece or wire in the tubing where your going to make your bend and pull it out. Then it is the same as for elbows and finish as above.

(2) Plastic/Styrene Tubing
Plastic tubing can probably be found almost anywhere. Usually the best place to look is at a hobby store. For making any bends such as the elbows or for curved tops or bullhauler tops, you can use a candle. Mark of your place where you want to bend your stack. Hold that spot over the candle about a 1/4" to a 1/2" of an inch above the flame. Don't hold it too close and heat it too fast because the plastic can melt or turn black. After a couple of seconds the tubing will start to bend down. Then you can finish bending to the correct bend needed. Then you can use a bench grinder or any other grinder or file to sand down the ends to the correct shape or form. To finish you can either use bare-metal foil or chrome bumper spray paint, chrome tape probably won't work on the bends.

 

(3) Brass Rods

Brass rods can be found at the same place as the aluminum tubing. Brass rod is quite easy to work with. What I do for a beveled stack is cut the rod to length, plus about another 1/4" or more for the elbow. For the elbow, I clamp it into a bench vise and use a rubber mallet to bend the end to make the elbow. It is best to place something soft like a cloth or towel around the rod before clamping. Then I take it over to a bench grinder to make the beveled top. For curved stacks, its the same thing as beveled stacks. Once making your elbows flip it over and use a hammer and bend the end halfway and then grind down to shaped the curved end. Finishes include bare-metal foil or chrome bumper spray paint. Chrome tape probably won't work for the bends.