Seasons 1 and 2 are out of order for It's Always Sunny. After Episode 4 of Season 2, it's all OK. This show doesn't have too bad of spoilers if you watch it out of order - but there are definitely some developments and references between the episodes that wouldn't make sense.
Season 1: 1,3,6,4,5,7,2
Season 2: 1,2,4,3,5-10
Season 3-7: In Order
Season 8 is available in HD on Amazon Instant Video - but it is not free for Amazon Prime members. Here's an affiliate link if you'd like to support the site, anything you buy on Amazon within 24 hours of clicking the link counts, and we appreciate it!
Season 8, Episode 1 [HD]