To get an exact replacement you'll need the following dimensions:

A - Number of splines (best way to get those is to take a clear image of the end of the spindle on your phone and then blow it up so you can count half of them and then just double the number. The most common spline 'counts' are 18, 20, 24 & 28 WRITE IT DOWN!

B - The spline diameter is vital. If you don't have a vernier gauge then wrap thread around it 10 times & cut the ends so they meet. Lay thread out straight (not stretched) & tape ends down. Measure its length & divide by 10 for the spline dia. Common ones are 7.6, 7.8, 8.0 etc. Don't be put off by apparently odd numbers. WRITE IT DOWN!