The ratio between the focal length and the aperture (diameter) of a lens is called the f/number. The smaller the f/number, the more light is let in. Fast lenses start around f/2.0, and the light let in goes as the inverse square. Compared to f/2.0, f/1.4 lets in twice as much light, f/1.0 four times, and f/0.71 eight times. The fastest camera lenses designed for DSLRs and widely available are between f/1.4 and f/1.2, but lenses as fast as f/0.75 have been made in quantity for special applications, and some of those are available quite cheaply via scrap yards, surplus stores, or eBay.
These ultra-fast lenses usually are branded either Kowa or Rodenstock and were designed for use in medical or semiconductor industry equipment, etc. They are not well-suited for use on DSLR cameras, and are no substitute for an f/1.4 or f/1.2 lens that was designed for your camera. However, they easily can produce very distinctive images. Here's how to use one on a DSLR....
Step 1: Get a Lens
Start by getting one of these ultra-fast lenses.
The example here is a Kowa 1:1 55mm, an f/1.0. There are lots of others to choose from, mostly branded Kowa or Rodenstock. The Kowa shown here cost $25, including shipping, via eBay. Personally, I've purchased ultra-fast lenses from two eBay suppliers: ctr_surplus and svcompucycle.
Fundamentally, these are commonly junked lenses and should not be too expensive. However, these lenses were used in very expensive equipment and probably cost a lot when new, and people often go nuts when they see fast apertures, so pricing varies wildly. The Kowa 1:1 55mm is among the cheapest and most commonly available, but there are many alternatives. I've heard that one can find ultra-fast lenses for $1 in junk yards if you're diligent and dress like a buck is all you can afford. However, surplus stores and eBay sellers are commonly listing ultra-fast lenses anywhere from $10 to $400. Some of the more exotic ones, or ones with DSLR-compatible mounts, are between $600 and $2,000.
In any case, you want a lens with a rear element that fits inside the diameter of your DSLR's mount. A lens with a larger rear element generally might imply vignetting, as well as being harder to mount on your DSLR. Independent of the rear element diameter, large diameter lens barrels can complicate mounting by colliding with an overhanging finder or a front hand grip on your camera body.