Basically if you do not have standard errors then there are a number of options.
1 it may be possible to back-calculate them from significance tests (or $p$-values) and the sample sizes.
2 it may be possible to impute them from similar studies which used the same measure under similar circumstances. In this case it would be wise to do a sensitivity analysis using a variety of imputed values. Mark the imputed values in some way if you provide a main forest plot.
3 authors of the primary studies may be contactable and may be willing to supply the missing values if you explain why you need them. If they do be sure to acknowledge their help in your paper and flag the results they supplied.