I have a huge fitting catalog in front of me. Closest they have is 1/2" tube to 1/4" NPTF then you'd have to run a 1/8 male to 1/4" female on the bag and connect the 1/2" Tube to 1/4" NPTF to that. Both fittings will be tough to find.
I think you are going to be better off finding a bushing to go from like 1/2" to 1/8" at the valve, than a 1/8" fitting in the bushing and at the bag. SD has both:
http://www.suicidedoors.com/12x18ReducingBushing.php
http://www.suicidedoors.com/18ODx18NPTFMaleElbow.php
no matter what you do its gonna be really fuckin slow with 1/8" ports.