There are six ancient accounts that deal with the battle:

• Herodotos 7.198-238 (written ca. 425 BCE)
• Diodoros of Sicily 11.3-11, based on Ephoros (the latter was writing ca. 350 BCE)
• Justin 2.11, based on Trogus (the latter is 1st century BCE)
• (pseudo-?)Plutarch, On the Malice of Herodotos 865a-867b (probably roughly contemporary with Plutarch even if it isn't by him, i.e. ca. 2nd century CE)
• pseudo-Plutarch, Lakonian Sayings 224f-225e (definitely not by the real Plutarch; date unknown)
• Ktesias, Persika (contemporary with Herodotos), as reported in Photios Myriobiblion cod. 72, 37.i.23-40 (= Fragmente der griechischen Historiker 688 F 13 section 27)

They're all Greek except Trogus (who was Roman), and arguably Ktesias (Greek, but maybe drawing on Persian material). The main ones are Herodotos and Diodoros/Ephoros. They disagree on some key points: for example, in Herodotus, Leonidas gets killed, the remaining Greeks form a shieldwall on a hillock, and are slaughtered. In Diodoros/Ephoros and Plutarch, the Greeks make a night attack on the Persians in an attempt to assassinate Xerxes, and are slaughtered. The two are in principle reconcilable I suppose, but Herodotos pretty clearly seems to set the end of the battle in daytime, so reconciling them seems misguided. Both play against models familiar from heroic epic.

The last source, Ktesias, is the least trustworthy, but is nonetheless very early and may contain some unique information. He drew directly on Persian sources, but his entire treatment of the battle of Thermopylae is wildly unreliable. He has the Persians attack with 10,000 men on day 1, then 20,000 on day 2, and 50,000 on day 3 (this rising tricolon is a folktale motif, so completely ahistorical), and all three fail; and he has the battle end with the Spartans being surrounded and slaughtered (i.e. like Herodotos). But on other hand, only Ctesias gives names for the Greeks that guide the Persian forces over the mountain pass (Thorax of Thessaly, and Kalliades and Timaphernes of Trachis): is he at all reliable on these points? Probably not, but we don't have anything better.