What replacement florescent lamps can I use?
Narva Specs
LT 15W/530-030, 15W, Warm White 530 color rating, 438mm long, G13 Socket, 26mm dia.

Satco Specs
F15T8-530, 15W, Warm White 3000 Kelvin, 451.61mm MOL, G13 Socket, T8 1"/25.4mm dia.


So, we have a lamp with the following specifications:

T8 15 Watt 1"/25.4mm dia tube with ANSI G13 medium BiPin socket having a 3000 kelvin Warm White color rating and a CRI (Color Rendering Index) of 50 that is nominally 18"/455mm long.

So, basically look for the following tubes and check the length. Narva measures from end cap to end cap while Satco uses Maximum Overall Length which includes the pins in the length.

Common designation
F15T8-530
F15T8-WW

Philips
F15T8/WW/ALTO

OSRAM Sylvania
F15T8/WW