Honestly I would have to agree with Sloppy Robot. My preference is to either buy an out of the box, entry level track bike or build one up with a proper track frame.
Really there are three schools of thought I've noticed...
The surgeons: Those who like taking Road Bikes and chopping / swapping and replacing. Usually because they "typically" use economical parts and existing parts on the bike they can do a track conversion for cheaper than buying a traditional track bike.
The Contractors: They buy Track Frames and build them up part by part to suit their taste and their budget. This is usually the most expensive route, but the contractor always gets the bike exactly the way they wanted.
The Consumers: They buy completed track bikes. There are several low end track bikes that people can start and build upon. This is a good choice for those who want the track specific frame and perhaps want to try it out, and can upgrade as you go on.
There also are plenty of expensive track bikes as well, that high track nuts can by "out of the box"
So depending on whether you just want to TRY fixed....or whether you already know fixed and just want one of your own...will determine which route you take...
Personally I like the personalization of building up the bike from the ground up. Even after years, I still try and find things I can update and upgrade.