What size tank do you have? Figure 8 puffers are a commonly available BW puffer that will do well with a SG of 1.005 to 1.008. They also grow to 3". Green spotted puffers are another commonly available BW puffer but they grow to 6" and will require high-end BW/marine water conditions(SG around 1.020) as they mature.
Figure 8's require 15 gallons/ fish so if your tank is at least 30 gallons you probably can keep 2 in there. Of course each puffers disposition varies so it is not always a given that they can coexist peacefully for life. The key to keeping puffers together is to have plenty of decorations to break up the lines of sight. Provide each fish with a 'home' that it can retreat to if needed.
Keeping BW is not really more difficult then FW. You do the same maintanance that you would use for a FW tank although with puffers it is recommended to do 50% weekly water changes since they are a messy fish. Puffers also do not do well in an uncycled tank so it is best to have a cycled tank to put them in. If you have another tank already you can seed it with bacteria from that tank.Most figure 8's are sold in FW at the pet stores so your tank should be set up accordingly. My LFS actually sells them in BW so you will need to double check with your shop.
To setup a BW tank you really only need 2 additional items: marine salt mix and a hydrometer or a refractometer. Hydrometers and refractometers measure the salinity/Sg levels of the water. Hydrometers are cheaper and tend to be more inaccurate. They run around $10. Refractometers are more accurate but run $40. If you get a hydrometer make sure you get one that reads from a SG of 1.000, since some don't start the scale until 1.010 which will not do you any good for a low-end BW setup.
Once your fish is home and you start doing the weekly PWC's you can start raising the SG of the tank by mixing your new water with the salt mix. You do not want to raise the SG more then .002/week because it might kill your biofilter and/or fish. You will need to mix the water in a bucket and mix it for a few minutes and then check the SG. An approximate ratio of salt/water would be about 1 tablespoon/gallon of water which should give you a SG of around 1.002. Temperature should be anywhere from 78 to 84 degrees.
Here is a link to the figure 8 puffer for more info on that species.
http://www.pufferlist.com/puffer/brackpuff.php?puffid=13
