Allergy to folic acid alone is quite rare. Folic acid is present in the diet. In true folic acid allergy, the patient should experience it with some folic-acid rich diets also.
When a multi-ingredients product, such as B-complex, is taken, one is never sure about the offending constituent. For instance the allergy could be due to vitamin B-1 (much more common compared to folic acid). Therefore very low dose folic acid, say 1.25 mg or one-fourth of a 5 mg tablet alone should be taken to see if it causes allergic reaction.
Folic acid supplementation is required to prevent neural tube defect in the baby and hence is very important before conception and during the first three months of pregnancy. Actually the requirement is quite small: 0.5 mg is adequate though most commercial preparations contain much more.
You can try a low-dose, slow-release preparation such as Fecontin-F Continus (ModiMundipharma) that contains 0.5 mg and iron. In case this also results in allergy then there is no alternative but to depend on dietary sources: kidney, nuts and leafy green vegetables. Please keep in mind that deep cooking destroys folic acid.