How bloody typical. Fella ask's a question - three responses, and not one of them come's close to answering the question.
I've had an ask around, and unless you were to "illigitimately aqquire" one from someone in say engineering, it could be a bit difficult to actually get a hold of one.
If you were to magically acquire one and take one into a public pool, just your luck some nosey type would take issue, and worse case scenario you'd find yourself having a sit-down with someone in regards to the possibility of interfering with aircraft equipment (or whatever the individual country calls it) which is illegal, and rather heftily punishable.
Cheapest bet - buy a standard boating life jacket from a sports or boating store, preferably one of the U shaped ones that clips around your back (similar to the standard aviation lifejacket!). Swimming with one of those on would be essentially the same as an aircraft one.
Most airlines training requires practical work in a pool, most of which is done whilst wearing lifejackets. So contrary to the 5 cents worth in the three posts above, if your not 100% comfortable in the water, then a lifejacket may make things a little bit more difficult, and some practice wouldn't hurt.
Hope that helps!