So she isnt your biological mother but she is the closest thing to it. Maybe she is your step mother, or God Mother, or maybe just your aunt. Either way show that you appreciate her on mothers day.
Even if your real mother is also in the picture, they both deserve to be valued. There is a way to make time for the both of them. You could also bring them both together just to avoid conflict.
A card or a blank note expressing your feelings is always a good gesture
Just a simple phone call goes a long way also
Lunch or a movie is an affordable idea as well
Flowers is a great gift but if that fails you can also make a gift
Baskets and home crafted cards are always nice and sincere gifts