Hi Megs,
Just visited your blog, great posts so far.
As for cheap blazers... I don't know which field you are in but let's just assume it's a conservative one. Typically for job interviews I think it's always best to try for a matching suit so I don't know if you want to wear your pants while you go shopping to try to find a close enough match.
Unfortunately I haven't seen any great sales in stores recently (the AT 40% sale isn't good enough to mark the blazers down to below $80) but definitely try stores like LOFT or even H&M (look for cropped lengths since you are so petite) which are budget friendly brands to begin with. Or, I don't know where you are located, but Talbots clearance centers have VERY good sales (routinely additional 75% off already reduced prices). I have seen very nice full suits there in sizes 6P-10P get discounted to $30.
Or, if all fail, I would buy the best fitting blazer you can find in stores (regardless of the price), take it home, measure it (shoulder, sleeve, length, etc.), take down the measurements, return it in perfect unworn condition and go on ebay. Sellers have to provide you with measurements (but be very specific and ask them to measure precisely where you took your own measurements). I think you can easily find a nice, brand new suit for under $80 there.
Good luck to you!