The needle taper is the most common part to change for mid-range other than the pilot and main jets. You don't need to change the needle jet unless you can't get the proper needle taper to do the job. There are some different slide cutaways available too, but you generally don't need to change them if it picks up well off of idle. Mikuni slides can be a bit pricey at about $30 a pop. Jets and needles are more reasonable.
Sudco is a big dealer but they mostly sell quantities to dealerships. motorcyclecarbs.com handles small orders but you pay the price.