If you got pregnant with clomid, the chance of having twins isn't that high. And I would ditto what adgal is telling you. there's no for sure process out there to get you pregnant with twins. Maybe IVF but you'd have to have super healthy embryos and convince your RE to put more 3 or more embryos in. And most RE's won't transplant more than 2 healthy embryos in at a time. But you do have a 50% chance with IVF and super uber healthy embryos.
Kelly is right, Dr.'s aren't keen on helping you conceive twins. Even with IVF, the goal is one healthy baby, although multiples happen more often with IVF. As for if chlomide is suitable for you, that is a decision you and your Dr. should make together. Chances of twins with chlomide are only slightly increased, most women still only conceive one. In my case, chlomide actually prevented me from conceiving (that happens in about 20% of cases). Please do not self prescribe. I know it is available for order on the internet, but you do not know what you are really getting. If you want to try it, talk to your Dr. and get a proper prescription.
Clomid is for people whom have problems ovulating on their own. It doesn't appear to me you have this problem. I don't think any Dr. is specifically going to prescribe you clomid to help your odds to conceive twins. The older you get it increases your odds a bit or if they run in your family.