The snow wolf m24 mags will work. I have 4 and they work in my javelin m24. I got mine off acmgear I think but that site is long gone. I think these are the same.
http://airsoft-club.com/shop/magazin...4-sniper-rifle
I'll dig around and see if I have a package for one to confirm model/part number.
They are shit quality plastic though, as is the original javelin one too.
As for the modify ones, not sure but much more monies over the cheap sw ones
Huang has the sw ones for $10. Might as well go there
http://airsoftdepot.ca/catalog/produ...sid=ojmjzkolyy