เอา version ภาษาอังกฤษเลยละกันครับ
1. Yes, you can buy a SIM card without the phone from either AIS and DTAC. Ask for post-paid SIM type if you don't want to pay monhthly fee: 1-to-call from AIS and D-Prompt from DTAC. The expiry date and talking time can be easily purchased.
2. DTAC is better in terms of price and value for money. AIS boasts better coverage especially outside Bangkok. If you're planning on trips in upcountry, 1-to-call from AIS would be a better choice. AIS packages however are somewhat more expensive than offers by DTAC.
3. The best place for SIM card purchase is perhaps MBK Center, which is already quite convenience, and you can even choose an easy-to-remember number (at a fee). Both operators (AIS and DTAC) can be easily found everywhere in Thailand.
Please stick with either AIS or DTAC, as other operators may offer better deals, but the coverage and outlets are far worse than the big two.