This is NOT Miss-able. There are 8 Summonable Characters in total
You can find the following for sale in Reptillio (Town north of Saronia)
-Chocobo - 100 Gil
-Shiva - 7000 Gil
-Ramuh - 7000 Gil
-Ifrit - 7000 Gil
-Titan - 7000 Gil
You will obtain Odin after defeating him in Saronia Catacomb
You will obtain Leviathan after defeating him - Lake west of Castle Argus
You will obtain Bahamut after defeating him - South East of Viking cove.