I'm a certified breastfeeding counselor and a Registered Dietitian specializing in infant nutrition. Personally, I'm also the mother of a wonderful 11-month old boy who is exclusively breastfed. Congratulations to you on breastfeeding for 9 months!
First, I wanted to comment on your daughter's growth--what you've stated is perfectly normal for a breastfed baby. Exclusively breastfed babies grow very differently from formula fed babies. The World Health Organization actually has different growth charts for babies that are breastfed. Breastfed babies tend to gain weight rapidly during the first few months of life, and then they gain weight very slowly after that. Basically for your daughter, as long as she's not losing weight, and she's thriving in other ways, she's fine!
Like many moms, I also experienced a decrease in milk supply between 6-9 months. I think it's simply a result of babies eating more solids and not needing as much breastmilk. I work full time, and I usually pump twice a day at work. I was able to increase my milk supply by doing a little more pumping at home. After I put my baby down to bed at night, I would pump. Not much would come out, but I was stimulating my body to produce more milk, which is the important part. On the weekends, I would also pump after my baby's first morning feeding. The key is just to get more stimulation so your body will make more milk.
Different things work for different moms; just try what you think you're capable of and know you've done such a wonderful thing for your baby to breastfeed for 9 months. Best of luck to you, and hopefully you won't need to supplement with formula, but it's not the worst thing in the world if you have to. It can be difficult to get your body to produce as much milk from pumping as just putting the baby to breast.