How to find the right battery for my project
There are only a limited number of battery chemistries and each one has its own voltage so there are only a limited number of voltages. Also, battery voltage drops as they get discharged so you won't find exactly 12V. Search radio-controlled hobby websites.
The most likely chemistries available to you, in order of increasing cost, increasing performance, decreasing weight, and decreased tolerance to electrical and mechanical for abuse, are:

lead-acid
NiMh
Lithium-Polymer or Lithium Iron Phosphate (or the other lithium
variants, just not lithium-ion...too low power to move a robot)

You will need a specialized charger for the better types of batteries such as Lithium Polymer and its variants such as Lithium Iron Phosphate since they real fire hazards (due to their high energy density) if mishandled, both electrical and physically.
A battery's energy capacity is rated in Amp-hours (Ah).
A battery's power capability is a C-rating and tells you how much current the battery can supply relative to its Amp-hour rating.
7Ah means (theoretically) it can provide 7A for one hour before it is dead. Drawing 7A out of a 7Ah battery is a discharge of 1C, and it should run for 1 hour.
If that same 7Ah battery is rated for 3C, then that means it can supply a maximum of 21 Amps. Drawing 3C worth of current from that battery will runs it down three times faster, so it will only run 1/3 of an hour.
So the fact you say you need 12A only tells us the power you need the battery to supply, not how long it needs to run for.
$$ Max.Amps = C.rating \times Amp.hours$$
$$ Run.Hours = \frac{1}{discharge.C.value}$$
$$discharge.C.value = \frac{Discharge.Current}{Amp.hours}$$