IMO, game production adds these LCs to an 'availability pool', when a player tags the 'Create Landing Craft' button, i'm thinking that LCs from the pool appear on a first-in/first-out basis - that is, the purchased LCs appear in the order in which they were produced. presumably when you disband them, they'll be added to the queue (after 30 days) in the order in which they were disbanded.
i've only gotten LCMs & LCVPs so far, and IIRC LCTs can also be purchased this way - but anything sizeable (LCIs, LSTs, LCI(G)s, LSMs, etc.) only enter the game as normal scheduled reinforcements.
i've found the little guys useful, shuttling supply & SeaBees around the southern Solomons. it's a bit ughfortunate that i can't choose which type i'd prefer to buy.
playing DBB_B, there's quite a few more little ships available such as YO & xAPc types.