Regression requires two types of tests: f and t tests.
To find the p values for the f test you need to consult the f table. Use the degrees of freedom given in the ANOVA table (provided as part of the SPSS regression output).
To find the p values for the t test you need to use the Df2 i.e. df denominator. Df denominator is specified in the ANOVA table output as mentioned above.
Note: SPSS reports one tail F test values (f test is always an upper tail test!), and it reports two tail p values for T tests. In case of simple linear regression – you will always find that both one tail F test p value and two tail T test p value are the same!
Alternatively, you can compute the degrees of freedom as mentioned here:
Df numerator = number of x variables in the model
Df denominator = sample size – number of x variables – 1
Df total = sample size – 1
Note df1 is also called df numerator, and df2 is also called df denominator