Has anyone had to switch to a different research/study focus to obtain an RA postion? To give a bit of context: I got accepted to UIUC for the Fall 2015 semester to start my graduate studies in Mechanical Engineering. I've been interested in Fluid Mechanics and Thermal Sciences as a field to specialize in for quite a while now. As such, I've been contacting many professors in that area to see if they have RA positions, but all have not been fruitful (no open slots). Having some sort of assistantship is the only way I can pay for tuition there without resorting to loans, which I really want to avoid since I was able to do so for my undergraduate studies. There are still professors I can contact in the area, but the number of options is dwindling. I've considered looking into the other departments in hopes of finding something before the Fall semester starts. I would like to "stick to my guns" on this because it is an area that I am actually interested in, but time is catching up with me quickly. 

It says on the MechSE website that 85% of students receive funding this way, so it seems like I would have a good chance of getting something. Should I be worried that I don't have something definite at this time? Most of the professors have been able to respond quickly (less than a week sometimes), but some simply do not respond. I'm not sure of what to do except keep emailing professors and consider my other research options.
There may be general assistantships available outside your department that could serve as a stop-gap until you can get within-department funding. The first year of my PhD I got funding working under a Dean in the School of Graduate studies. I also was a library GA during my master's. While funding specific to your degree is ideal of course, there are some fringe-benefits to some of these other positions. The library position allowed me to do my own work for 15 of my allotted 20 hours, and the Grad Studies position gave me a ton of insight into the inner-workings of administration that I may not have gotten otherwise.