L.S.
I don't know of any kind of supplements to increase fertility, but I have to pass on the advice to read Toni Wechsler's book "Taking Charge of your Fertility". I've read it about 4 times, and successfully identified our fertility problem before we needed a doctor. In our case, we just weren't in the same city on the right days, and for our 2nd, my fertile period was very, very short.
The book outlines a basic way to understand whether you're fertile, by just being aware of the consistency of your cervical fluid, and (if you really feel like it), charting your temperature every morning. It's pretty awesome, and way cheaper than ovulation kits, and a lot less nerve wracking than tons of tests. Try it. Then, once you have a few months of charts, go see your doctor & see if you really need to increase your fertility.