As Leon mentioned, Colombians (Bci) can be a good boa to try. They can reach an impressive size though, so only if you are prepared to house, feed, and handle something like that. They have incredible strength, and a grumpy one can be a handful :p $100 and up.
6' female
<img src="http://www.ssnakess.com/photopost/data/508/22chas1-1m.jpg">
Other localities of Bci are available as well, such as Hog Isle boas, Cay Caulker, Crawl Cay, Mexican, etc. These relatively have the same care requirements, but have variations in appearance and size. 6.5' female Hog Isle is pictured below. Some of these tend to be a bit more pricey than regular Colombians however. $100 and up.
<img src="http://www.ssnakess.com/photopost/data/508/22peanut-01-med.jpg">
Rosy Boas are fun little boas to keep. They stay small (2-3 feet) and are on of the few boids that actually thrive in aquariums, if you don't like the looks of rubbermaids. They can be housed in a ten gallon for their whole lives. They are very deliberate in their actions, and are mostly docile. They come in a variety of different colours and patterns as well. $125 and up.
Mexican female
<img src="http://www.ssnakess.com/photopost/data/508/22sweetnessinhand.jpg">
Whitewater Canyon female (this girl always seems to think she can eat me, it's too funny :p, she never strikes at me, but slowly noses in and tries to gulp me down)
<img src="http://www.ssnakess.com/photopost/data/500/22rosyboa.jpg">
Sandboas can also be good, but make sure you get a well-started animal. Sometimes they can take a little bit go get feeding. There are several subspecies, the most common and affordable being the Kenyan. They stay small (1.5-3 feet) as well, and have similar care to rosy boas. Their normal phase is orange and dark brown splotches, below is the axanthic version. $60 and up.
<img src="http://www.ssnakess.com/photopost/data/508/22wee_axanthic_kenyan.jpg">