If you're correct about 70 feet being the usual casting limit for most casters using double tapers and shorter rods - and that sounds about right to me - then realize that we needn't go to unnecessary lengths (pun unplanned, but appropriate) to find spey-length double taper lines. Most DTs are either 82 or 90 feet, so they'll serve just fine as is. And they're a lot easier to buy on the cheap. Last year, I bought a group of three Shakespeare DT10F 90' lines on eBay for $6.
They're intended for splicing, but could be used out of the box.
And it's easy to enhance a standard double taper. Forty feet of running line (left over from that old WF line you saved or cut up) spliced onto the back end of an 82' DT makes it a 122' line. Want to use tips? Cut off the front taper (ca. seven feet), add a loop and a 15-foot sinking tip, and you have an inexpensive, versatile 130-foot line.