You can call DirecTV and request to outright purchase any receiver. The pricing might be a bit hard to swallow (for example, the last I heard an HD-DVR like the HR20/21 was around $500) - and you might have to play a bit of CSR roulette to find one who actually knows how to do it - but you can purchase them.
They don't want you to - and they make it difficult to do. But it can be done. -- The inherent vice of capitalism is the unequal sharing of the blessings. The inherent blessing of socialism is the equal sharing of misery.