The two most common subspecies of Boa constrictor are imperator and constrictor, BCI and BCC for short.
South America is home to the larger boas. There are many subspecies found here, Amarali, Longicauda, Constrictor, Occidentalis, and Ortonii (which is not available).
Mexico, down through Central America and into Western Colombia is home to BCI. As well as numerous islands off the coasts of Belize, Honduras, and Nicaragua.
Some of the BCI locales are very small boas. A female Tarahumara Mt. Boa from the Sierra Tarahumara in the Sonoran Desert, Mexico, reproduced at 38" in length. So if you want a truly small boa, a Tarahumara would be the ideal choice.
Here are two examples from a pure line produced by Gus Rentfro of Rio Bravo Reptiles.


Pure Tarahumara Mt. boas are however EXTREMELY difficult to find.
Most insular BCI are fairly small. Cay Caulkers, Crawl Cay, Hog Island (Hog is spelled with one G, two Gs are often used with non pure animals), and Corn Islands.
There have been some decent sized Hogs, but I expect most were crossed at some point. As well, overfeeding can produce an artificially large boa.
Mainland small BCI locales include Sonoran and other Mexican boas, Honduran, El Salvador, and Nicaraguan. Few pure Honduran and El Salvador lines exist. Many Nicaraguans are often sold as Hondurans, and within the US most El Salvador are from the Blood morph project (of which many today are not pure).
I have found that the Costa Rican and Panamanian boas I would not consider dwarfs. They have smaller body structures than say Colombians or BCC, however they are decently long boas.
Most boas are of unknown locality, or are crossed. There are websites that detail all the known boa morphs, and relation to different countries of origin.
The most common boas are Colombian.
While there was an Albino Sonoran, as well as an Albino Chiapas, Mexican boa, these are not available. Colombian Albinos are very common. Some breeders have produced Albinos with varying degrees of Cancun, Honduran, Mexican, Panamanian, El Salvadorian, and Nicaraguan blood. There are two Albino strains, Sharp and Kahl, which are genetically incompatible. If a strain is not mentioned, it is Kahl. Be weary as recently some people have tried to pass Kahls off as Sharp strain to unfamiliar buyers.
Example of Sharp strain:

Another subspecies to consider would be Sabogae, found in the Pearl Islands and other Islands in the Bay of Panama. Again, caution should be observed as true pure examples are rare and expensive.
If you want a small boa, there are many options. If you want an Albino, there are many options. There are still options for smaller Albinos, though they are limited and will still obtain a decent size.
If locality and predictability on how the boa will mature, act, and breed are important, it will be more difficult to locate what you want.
What size of a snake are you looking for? Other than Albinos, is there any other color morph or natural phase you like? The various subspecies and locales create a wide array of phenotypes without mutation.
Also keep in mind that one kind of 4 or 5' boa is going to be very different from another. A 5' Costa Rican male may only be 2 lbs. Where a 5' Colombian female may be 8lbs.
An example of said CR:

Those are two extremes for reference of a boa of the same length. Some locales have more arboreal tendency resulting in slim body structure, others are a little more heavily built. In general a healthy boa should have a lean body shape.
Hope this helps, please feel free to email me at chris.gilbertboa@gmail.com if you are have any off forum questions.
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Gilbert Boas
The Boa List!