The high C on the TI JF346 set is 0.033, and is the smallest JF string that they make, so you would be on your own for a high E.. The good news is that that string might be plain steel and not wrapped, since you might be looking at a 0.028 or something. I wouldn't sweat the halfstep detuning to Eb. The tension will be lower but it shouldn't be a problem. Tuning a BEADGC set up to Eb will result in broken strings and/or a broken bass. Not the way to go.
FWIW, I play a 5, also fretless, tuned EADGC, and that high C is a 0.027 roundwound nickel string offerend by Zon. I buy singles to fill out a light gauge 4 set. The whole set runs: 095, 075, 055, 035, 027. That set is pretty darn light, and since you are looking at Eb tuning you could bump up to a 0.100 set (or heavier) and be looking at something like 100, 80, 60, 50, 35, 25. They aren't TI Flats, but check out Zon at
http://www.zonguitars.com/ and click through the Shop Zon, then Shop Online. Their nickel strings are excellent and they offer everything in singles. You could also buy a stock 4 string set and then fill out the high strings from the singles. I think the smallest they make is a 0.025. Might just do the trick. Also check out their piccolo sets. The highest strings are plain steel, but they run down to 0.018!!!
I use a LOT of TI strings, but honestly TI is not great for "out of the box" tunings. They only offer one gauge for each type/scale set, so you can't cherrypick between part of a medium gauge set and part of an extra light set.