I understand how you might feel a little awkward about it, but fear not! Most folks on Ravelry are sympathetic to such problems and will help if they can.
I had a very similar situation to yours with a sweater. I was positive I had plenty of yarn -- until I got about halfway through and then realized I was going to run just a little short. I found two people on Ravelry who had the exact yarn I needed (even in the same dye lot!) and sent them each a PM asking if they could spare any and letting them know I'd be happy to pay.
They both responded very kindly and offered me their leftover yarn. I ended up getting just the yarn I needed, and the wonderful person who sent it to me refused money! Thanks to her I was able to finish my sweater.
** My poor neglected blog is here
RIP Gabby (9/12/2009) -- I miss you SO much. We'll never forget what a sweet girl you were!