>>I was wondering where can I get a new jet<<
The 1976 Batavus used an Encarwi Carb, so any of the places listed in the "Resources" section of this site should know where to find one.
>>how much will it be<<
Anywhere from $3.95 to $8.95 - all depends on who ya ask and how bad they wanna gouge you for it, most shops gouge on jet prices.
>>and how do I install it in the carb?<<
Unscrew the old one (it's mounted in the side...I'll paste a link to a carb diagram at the end of this post) take the old jet out of the holder, put the new one in the same way and screw it back in, simple as that....and with most Batavus you don't even have to dismount the carb to do it...tho you might have to loosen it at the intake and roll it sideways slightly (don't forget to re-tighten when yer done, or you'll get a vaccuum leak problem...superfast idle, etc..)
Carb Specs for the carb I think that moped has are here.
It's the top diagram, and the part in question is 16/17 - 16 is the jet holder, that just holds it in place...17 is the actual jet itself.
Hope alla that helps some,