Our team is using the Versadrop from VEX http://www.vexrobotics.com/217-4824.html
Pancake Pneumatic Cylinder, 32mm Bore, 15mm Travel
I am not sure what kind of fitting I should use for that kind of cylinder.
Can anyone provide some sources about pneumatics fittings? Thanks.
VexPro has the fittings for the pancake cylinder
The Vex web site says that they use 1/8 npt fittings. So you should use 1/8 npt to 1/4 tube. Lots of choices. We use Automation Direct fittings. Buy them with your voucher.
These are not for the same cylinder. These are 10-32 fittings for the 3/4" bore cylinder used to actuate the shifting mechanism in the Ball Shifter gearboxes. The cylinder in the Versadrop modules is a bigger 32mm bore that uses 1/8" NPT fittings, per the “Product Info” tab on the Versadrop page.
I’ve found Hose and Fitting Supply to be a good supplier of pneumatic components. You can find the fitting page here: http://www.hoseandfittingsupply.com/push-to-connect-fitting-s/7.htm. Additionally, if you haven’t used the voucher already that came in the KOP, http://automationdirect.com should also have the fittings you need.
McMaster.
Just to add, Bimba is a good supplier for most teams.
But if you’d like to join the dark side, Festo is an amazing supplier and has conversion fittings from imperial to metric 
Make sure if you order the 1/8" NPT x 1/4" tubing connectors from Automation Direct, that you actually get what you ordered. I used the voucher and ordered 10 each of the 1/8" NPT x 1/4" tube straight and 90 fittings, and even though the bags were marked correctly, we got 1/4" x 1/4". Now we have to return them, explaining they are wrong, even though the bags are printed correctly. grrr…
As nice as Festo is, they’re hard to find, and quite pricey. Bimba’s Pneumadyne fitting line is alright, but the plastic they’re made of is intended for lighter duty applications. I’d go with Norgren’s C series instead, they are a lot more durable, and the straights feature an internal hex, which is a fantastic tool.