I will be flying into Istanbul in May and then from Istanbul to Dalaman. I was wondering if I should purchase my ticket to Dalaman before my trip; or will I be able to easily purchase a ticket when I arrive in Istanbul. Also, does anyone know the approximate price for this ticket? I was told that it is cheaper to buy a ticket once I'm in Istanbul. Tesekkur ederim

Check the THY site, I just looked and e.g. on May 15th a single trip Istanbul-Dalaman costs now 104 YTL including taxes. I don't know if that's more or less expensive than what you'll pay at your travel agency. But the longer you wait, the more cheap places are sold out.

I think you should buy it through Turkish Airlines because foreign agents sell turkish domestic flights at ridiculous rates.