they exist, in a sense that they were made (at least prototypes).
Albatros announced they would produce a "riser" adapter a couple of years ago, but it never went beyond the prototype stage.
An alternative is to buy a motherboard with both AGP and PCI-e slots. Asrock (subdivision of Asus) makes such mobos, but they generally use older generation chipsets so you will not be able to install any of the 45nm Core 2 Duo CPUs made by Intel. They will also not support any type of Quad Core CPU regardless if it is an older or new generation of the Core 2 Duo CPU.
More importantly the PCI-e slot is limited to x4 lanes instead of x16 lanes. While x4 lanes will suffice for entry level/mainstream card, having so few lanes will definitely bottleneck a 8800 GT or GTS (I think I've read a 25% performance loss).
My advice, ditch AGP and simply get a PCI-e video card.