I reckon a correct triple-length spindle to replace a FR spindle on a double might be as hard or harder to find than a complete BB unit: the tricky part is getting one with both the correct length to work with whatever crank you'll be using as well as having the internal spacing between the races to work with your FR cups. Personally, I'd say this is one where just buying the high-priced unit from Phil Wood makes perfect sense: you'll find something that fits and it will last.