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It'd be better if you can apply for a Tourist Visa before coming here. It normally takes 1 day to process in Thai Embassies. A Tourist Visa is good for 60 days, and then you can apply for a 30 days extension at the immigration office here in Thailand.

So with a Tourist Visa, that gives you 60+30 Days.

Then, when you've been here for 90 days already (before it expires) you go to a nearby border and enter/exit the country and they will give you a 30 Day Visa Exempt Entry Stamp. (but you can only have an aggregated stay of 90 days per 6 months in your passport using the visa exempt entry stamps)

So, if you are coming here without a visa, you'll be able to stay for about 90 days (considering you'll be getting a stamp when you enter, and then doing 2 times a border-run). Then if you intend to stay longer, you'll need to go to a nearby consulate or embassy to get a Tourist Visa. You can go to Singapore, Penang, Kuala Lumpur (popular choices) to go get a Tourist Visa. Flights to these destination from Thailand are pretty cheap, around $100-150 for round-trip.

Check these sites for air-fares in south-east asia.

http://fly12go.com/en/main.shtml
http://www.thaiairways.com
http://www.tigerairways.com
http://www.airasia.com
http://www.jetstar.com