You can buy a power supply with a higher amperage rating with no problem. The pump will only draw the current it needs from the power supply. Just be sure the voltage and minimum current rating is ok. It appears to me that a 2 amp 12v power supply will work just fine.
It is quit okay to use a 12V 2A (2 Amp) supply. In fact it is better to have extra current available rather than use a 1.2 Amp supply since the bigger 2A supply will run cooler and arguably be more reliable.
Joe's Ancient Old Mead (JAOM) 10/26/2012
Apple Jack's Limeade Skeeter Pee
Skeeter Pee 10/6/2012
NB Cream Ale and AK47 Pale Mild
Switched mode or linear mode should be fine. Switched mode is typically more efficient. An old computer power supply may or may not work, some computer power supplies will not operate if say only the 12V is loaded; some of them are funny about wanting load on some of the other voltages as well from being plugged into the motherboard.