Salvia Divinorum is a plant made popular because of its hallucinogenic properties. It is also an attractive and fairly unique plant of the mint family. Most Salvia Divinorum plants are grown from cuttings or genetic "clones," and seeds are extremely rare, difficult to find, expensive and usually not viable. Be prepared to pay as much as £65 or more for a single seed with absolutely no guarantee of it being viable. It is very important to keep in mind that some seeds are fraudulently sold as Salvia Divinorum when they are not, because of how valuable these seeds are.
Fly-by-night Internet sellers of "new strains" and seeds of Salvia divinorum are surprisingly common. This is in part caused by the illegality of owning these plants in some states and countries. A reputable shop will not sell or ship to these states; so if a company is willing to do so, you can bet the seeds are fake. S. divinorum seeds are so rare as to be almost a myth--the plant reproduces vegetatively, rather than via seed propagation--but there are shops that do sell them, such as Daniel Siebert's Sage Wisdom. Be prepared to buy immediately and spend a great deal of money for each seed.
While most people acquire S. divinorum online, it is possible to get the seeds through a friend or acquaintance. Make friends with Salvia growers. Make sure that they know you are willing to pay top dollar for seeds. In the rare event that one of these acquaintance's plants sprouts seeds, she will know you want some and might be more likely to give you the pick of the seeds. This may be the best option because it will be harder for the seller to scam you if you know her personally and have seen her plants.
Having seeds grow on your Salvia divinorum plants may seem impossible, but the chance is, really, only very unlikely. If you have your own Salvia crop, you will have a large variety of plants with which to attempt to grow new strains. Instead of spending an exorbitant sum of money on a single seed, try buying a number of plants from as many strains as you can and growing them together in a greenhouse. Aim to buy only the healthiest plants you can from reputable sellers. There appears to be a slightly higher likelihood of pollination if you do it by hand. If your plants actually manage to produce seeds, you will be in the enviable position of one able to sell Salvia divinorum seeds. Or keep them if you wish, instead of spending money to buy them.
Get references for every seller before purchasing.
Never buy seeds from a seller you have not heard of. Don't be the victim of a scam.