I just made mine using an intake tube from a 4.3 chevy. GM p/n: 25176891
I buy one or two of these depending on how I want the air filter to sit inside the fender (you can get away with just buying one): GM p/n: 25176891 - GM Truck induction tube/duct w/ bellows and clamps:
Cut in half (roughly) and reassmble by putting the now cut-off elbow onto the rest of the section that has the bellows on it. The bellows connect to the throttle body. Here are some pictures:
You can get away with using only 1 induction tube (cut in half and reassembled) if you run a smaller open-element K&N air filter (p/n: RU-3130) as seen below:
If you run a larger filter, you may need to buy a second induction tube and cut the elbow off of it so you can orient the filter as shown below:
These induction tubes were used on certain 99-newer GM fullsize pickup trucks that had the 4.3L V6; they measure 3.5" diameter so they are good enough for most performance applications. You might get lucky and find some in a junkyard and save some money since the price on new ones from GM have really gone up the past few years.