Allegiant is a great example. They fly slightly less fuel efficient MD-80s which can be bought at bargain prices from the legacy carriers (MD-80's are those narrow body jets with the engines in the back and the 3-2 seating arrangement). They make it work by flying nonstop flights from smaller airports (without competing carriers on those routes), with low frequency. The low frequency also gives them longer until they have to do a heavy maintenance check -- which, for an older MD-80, usually costs more than the plane is worth. http://www.fastcompany.com/1325762/heard-allegiant-air-why-i... .
By the way, that's why we have this phenomenon: http://en.wikipedia.org/wiki/Aircraft_boneyard
a) Never take any planes out of service
b) fly every plane until it crashes
No manmade machines have infinite service life, so a is out. No airline passengers will accept b.
It's not like a car, where it can be driven until the wheels fall off. Planes are flown until they need an expensive scheduled maintenance check, even if they seem to work just fine.
(To be absolutely clear: I don't think this is a bad thing. Maintenance checks are good. Falling out of the sky is bad.)
Go look at their route schedule. Looks like penetration to me to more established areas.
It fits Clayton Christensen's disruptive model perfectly.
Boing and Airbus are the only viable suppliers of long-range commercial aircraft, and if you order a 787 or A380 you will need to wait[1].
But it's not "locked up" so much as it takes a fair while to build, and they do have order books. But they are desperate to sell as many as they can (stories of orders being cancelled aren't uncommon).
In the short and medium range market there are a number of other suppliers. Embraer and Bombardier both have planes that sell well. For the more adventurous there are the Irkut MS-21 and Sukhoi Superjet 100 (both pretty modern, but I think only in service with Aeroflot)
As noted elsewhere, there is a large market in second hand aircraft.
[1] There is actually http://en.wikipedia.org/wiki/Ilyushin_Il-96, but I think that has only been sold to Aeroflot and other Russian airlines. Edit: turns out the passenger version of this is now out of production.