1) Don’t have Seville oranges? Use naval or juice oranges - the juice just needs to be concentrated. As Rose suggests, butter a 4-cup heat-proof glass measuring bowl and add 1 c. freshly squeezed orange juice to it. Microwave on High until the juice reduces to 2 Tb. In the recipe, only use ½ c. sugar, instead of ¾ c. sugar.
OR, try 2 parts fresh orange juice to 1 part lime juice (Note: I have not tried this method, but it's worth experimenting).
OR, try Calamondin juice if you have access to that precious fruit.
2) Have too many Seville oranges? Juice them and place the juice in airtight containers and freeze for up to a few months.