substituting ammeter shunts
If 75 mV is developed across a shunt at 30 Amperes, then by Ohms Law the shunt resistance is $$R = \frac{V}{I} = \frac{75 \text{mV}}{30 \text{A}} = 2.5 \text{m}\Omega$$

So if all you can find are 50 mV shunts, then you might think that a shunt of the same resistance as above, with a nominal current of: $$I = \frac{V}{R} = \frac{50 \text{mV}}{2.5 \text{m}\Omega} = 20 \text{A}$$ should do the job.

The problem, of course, is that the 20 Ampere shunt is not guaranteed to survive 30 Amperes running through it.

Solution: 

Two shunts in series, of 1.25 mOhms each, of the 50 mV variety. For 50 mV rated shunts to be of 1.25 mOhm, the rating you would look for is 40 Amperes, 50 mV.

Two of those in series develop the same 75 mV end to end when 30 Amperes flow through them. Calculate: $$V = 2 \times I \times R = 2 \times 30 \times 1.25 \text{m}\Omega = 75 \text{mV}$$ Perfect. 

Best of all, those 40 Ampere rated shunts will comfortably survive a 30 ampere current, so you are home free.