First off, what classes are your retainers? Let's say if one of them is a Marauder.. and you leveled your Marauder to level 15, they'll only be able to reach level 15 as well.
As for the materia! You're best bet is to stand in the market areas of Limsa, Ul'dah or Gridania and ask in Shout if someone can help you with a meld. If that doesn't work, try asking for it in the party finder.

Hope that helped!