Personally, the calculator I like most is the TI-84 series. There is no big difference between the different TI-84 models, all are good to use.
Where to buy:
Buying from school is the best option, as often there is a discount. Otherwise, you can buy from a distributor at Bras Basah Complex: Learning InterActive Pte Ltd The Resource Centre Blk 231 Bain Street #04-39 Bras Basah Complex Singapore 180231 (http://li.com.sg/html/li_contactus_details.html)
If your calculator is damaged, do not throw it away, as there is a chance of a trade in (pay around $90 to trade in the old calculator for a new one).