[Kindred]

Just make an input dialog whenever you want the player to search a player. For example, a command.

pawn Code:

#define DIALOG_PLAYER_SEARCH (1)

ShowPlayerDialog(playerid,DIALOG_PLAYER_SEARCH, DIALOG_STYLE_INPUT, "Search", "Type a players name to search it!", "Search", "Cancel");

Then OnDialogResponse

pawn Code:

switch(dialogid)
{
    case DIALOG_PLAYER_SEARCH:
    {
        //Do whatever you want it to do with inputtext, which is what they type
    }
}

You weren't specific what-so-ever, you didn't say you wanted it to check a database (mysql) or files for the name, etc, or if you just wanted online players, etc.