As you draw more and more schematics, you'll probably develop a preference towards uncluttered schematics. You'll find out soon that placing specific components for the supply rails clutters them too much. Even if it's just a single component for a dual split supply, like the one you're looking for.
To avoid this altogether, you can simply define these voltages directly in the same text box where your simulation directives resides. Like this:
V98 VS+ 0 15
V99 VS- 0 -15
** Your simulation directives follow (what's below it's just an example):
.tran 0 100m 0 1m startup
Then use the net names
VS- all over your schematic as you need them.
The reason for choosing V98 and V99 as names for the source (instead of V1 and V2, for example) is to avoid conflicts when placing source symbols in the schematic. This happens because the schematic editor in LTspice doesn't seem to "observe" device naming in spice directives.
(Thanks go to jonk for pointing this out in the comments, and making this answer a better one).