Okay, I personally think they are pretty similar. The only differences--sports jackets have a center vent, or opening in the back of the jacket. Suit jackets can have either one center vent, side vents, or no vents. Blazers usually have side vents. Blazers have gold (or silver, I think) buttons--kinda blingy. Sports jackets and suits have buttons that match the top. Sports jackets often (but not always) come in a bit different colour or style--suit jackets match the bottoms. Sports jackets cannot be double breasted, but blazers and suit jackets can be single or double breasted.
Whew. So I think really, for an interview, you're OK with your suit jacket top paired off nicely with a pair of pants in a different colour.
I know this sounds pretty cheesy, but did you ever try a thrift store for a sports jacket? For a lot of people, suits are not every day things--you know, you buy them for a nice occasion, wear them once or twice, and then you gain weight and can't wear it ever again so you give it away. I go to the Goodwill, and find lots of very nice, "special occasion" clothing there in practically new condition for pretty cheap. It's not going to be perfectly tailored, but it might come pretty close to what you need. Or you could check a department stores--There is a chain department store called Boscovs in my area, and you can sometimes get amazing deals there for very nice clothing. Good luck on your interview!