You have to have a minimum number of posts (10 I think it is) and then you have access to the Classifieds here. You will find domestic and international sellers there. The classifieds offer 'Trade' and 'For Sale' posts to acquire chips. Online marketplaces (craigslist etc.) are also productive on the rare occasion, but those are highly competitive and quality chips are found within minutes/hours of being posted so you gotta be quick.
You can also harvest chips directly from the casino, which is highly underrated in todays market as flavor of the month chips often have a market value of above face value for the denominations up to the $5.
Until you get to the 10 posts, spend your time looking through the various threads and galleries to get a feel of what you like. Once the classifieds open to you, don't let what other people think are desirable chips affect what you have come to admire. High prices do not necessarily equate to better chips.