How do we, as IT professionals, sell the importance of DR to senior management, given this is going to require additional funding and budgets that may not be available?
Firstly don't use the word "disaster". Management immediately thinks of natural catastrophes. Use the term, "IT recovery" but if you don't want to stray too far from conventional parlance, you could always go with..."IT disaster recovery".
Start talking about IT DR in terms of risk mitigation. Most senior executives understand the concept of risk very well. You can leverage this to your advantage by helping them frame up the issue in terms of risks they will pay to mitigate versus risks they are willing to accept.
Make sure senior management understands the benefits of IT DR, but use the benefits they're interested in, such as the following:
Gain competitive advantage
Increase revenue
Gain the ability to predict business performance
Meet supply chain demands
Meet regulatory and compliance requirements...