Fine alexandrites or alexandrite jewelry can be purchased from jewelers specializing in colored gemstones, at trade shows, from TV sellers, at high end jewelry auctions, or online from websites specialized in alexandrite. You would be unlikely to find a high quality alexandrite on Ebay but, you might find an inexpensive stone there.
Beware of anyone claiming to sell Russian alexandrites. There has not been any significant production of alexandrites for almost 100 years so the chances of someone having a real Russian alexandrite for sale would be very unlikely.
When buying alexandrite always ask for a Certificate of Authenticity or a Gem Identification Report from a reputed Gem Laboratory. Most reputable dealers selling alexandrite will be able to supply a certificate.
After you find the stone, look for a jeweler or a jewelry designer who can help you with the design and manufacture. Alexandrite is an expensive and rare gemstone, and should be set in 18k gold or platinum.
Recommended online alexandrite sellers: