How to calculate force without acceleration?

There are many ways to calculate force. However, they are completely different aspects. The force on an area is pressure, so F = p*A. In magnetic fields you use another formule (forgot the formule atm). Unless you define what kind of force you are looking for, I can't help you any further.

First of all, your question makes no sense. There wouldn't be any force if acceleration is zero.

Secondly, if you want to solve question in which acceleration is not given, you can use following formulas:
1.) F = dp/dt(dp = change in momentum, dt = change in time). In other words, rate of change of momentum gives you acceleration.
2.) Pressure = Force/Area
3.) Gravitational force = Gm1m2/r^2 ( G = universal gravitational constant, m1,m2 = masses, r = distance between their center.
4.) Magnetic force = q(vxB) [ q = charge, v = velocity of the particle, B= magnetic field].
5.) Electric force = qE [ E = electric field, q = charge]
6.) Force = surface tension x length. 

These are the most important one's there are many methods by the way. 

Hope this helps you and have a nice day/night ahead!