There are actually two sides to this discussion. Many people will buy a takedown bow with the thought that they would buy replacement limbs or limbs in other weights from the bowyer who made the bow. For some bows, like the Bear take down, you can find lots of these bows or the limbs on auction sites.
The other side is wht R H Clark is talking about. There are a number of bow risers that are designed to fit the ILF limbs or which can be retrofitted to accept them, like the Warf bows.
ILF (it stands for International Limb Fitting system) limbs are competition limbs that are very efficient and easy to shoot. Many people are also using them for hunting.
Hope this helps,
Larry