I went to the croissant shop which is really famous in sydney today. Althouth I arrived there around 12 p.m., most of them have already sold out. But I was lucky that I got last one of chocolate croissant. klingt das natürlich?

The first sentence is a bit confusing. "Today I went to a really famous croissant shop in Sydney." is, to me, easier to understand as otherwise I don't know if the shop is only really famous in Sydney, or generally really famous. "Although I arrived there around 12pm, most of the croissants had sold out." Fixed a typo, and I think the "them" was a bit ambiguous so I substituted "the croissants". Perhaps others might think that "them" is better. "But, I was lucky enough to buy the very last chocolate croissant." "I got last one" doesn't sound right to me, and "very last" helps emphasise the luck.