“I have a video card for my PC that is a PCI Express 16x form factor. Is it possible to buy an adapter to put the 16x card in an 8x, 4x or 1x PCIe slot? I don’t have a 16x slot.”
The short answer to your question is no, there isn’t a way to adapt a PCIe 16x card to a PCIe 8x, 4x, or 1x slot. That’s largely because using a 16x PCIe card in your computer is more complex than simply fitting it in a card slot and having it work. Each of the 1x, 4x, 8x, 16x and 32x PCIe slots actually transfers data to and from a seated card at different rates. A 16x card needs to have access to more data transfer than a 1x card, so it needs the larger 16x slot. A breakdown of the data transfer rates of each of the slots helps illustrate this:
PCI Express Bus Bandwidth
PCI Express is a serial transfer technology. The PCI Express transfer bus supports sending data in two directions at once. Standard PCI is a parallel transfer bus, limiting data transfer to one direction at a time. A 1x lane in PCI Express offers the potential to transmit in both directions at once. So a 16x PCI Express card slot is made up of 16 1x lanes all supporting 2-way data transfer. In the list below, the first number is the single direction transfer speed while the second number represents the bandwidth for transfer in both directions simultaneously. It’s also worth noting that PCI Express does not share bandwidth across card slots the way legacy PCI slots do, so there is less overall congestion on the PCI Express bus, resulting in better performance.
PCI Express 1x 250 [ 500] MB/s
PCI Express 2x 500 [ 1000] MB/s
PCI Express 4x 1000 [ 2000] MB/s
PCI Express 8x 2000 [ 4000] MB/s
PCI Express 16x 4000 [ 8000] MB/s
PCI Express 32x 8000  MB/s
Conversely, you can install a 1x, 4x, or 8x card in a higher performance slot, so it is possible to seat a 1x card in a 16x PCIe slot.