What is a good Time Capsule substitute?
Any Drobo product will do, but it’s possibly more expensive than a single Time Capsule. The advantages are a lot tho. :)

You can compare the different models. I have used a Drobo (not pro, not fs, not anything) and it had problems with certain reliability, but I’ve heard nice things about Drobo FS and even Drobo Pro. 

There are other NAS out there, so find one that you can afford, seek for reviews and go ahead :).

The nice thing about (Drobo in this case) is that you put “normal” drives in there, if one fails, you just replace it. BOom. That simple. It works in practice too. I’ve had two drive failures while using the Drobo.