It doesn't have to be a locksmith, just someone with an old fashioned type of cutter with the "exposed" cutting wheels. The "insert the blank here" cartridge key machine won't cut down the tip. The early keys are the same as a Miata, except that the Miata key has a long (unused) tip. With that removed on the old fashioned type of key cutter, it works perfectly in the Elise.The '05 Elise keys came in two varieties. The earlier keys were lighted with a little push button on the large plastic piece. The later keys were no longer lighted. If your key is one of the latter, then the X168 blank should work.
I would not let the guy at Lowe's cut the blank on their little insert the blank here machine if I were you. I'd have a proper locksmith do it. If the key is cut from the tip back toward the base it will end up really long and when inserted it will stick out a bit too far. A locksmith will be able to cut it from the base out and rid the excess length.