Both my wife's and my bio fins' spring straps have torn at the strap wher the finger loop is attached.
I bought a pair of Blue Reef SS spring Straps from Leisure Pro ($19.95) - removed the heel guards and put them on her fins (I'm not going to try this on my fins first!)
She has small fins and small Apollo Spring Straps. The Blue Reef fit perfectly and are MUCH less expensive than the Apollo replacements, plus they aren't as soft as the Apollos (last longer?).
There was a problem getting the screw off the straps as they are a few years old and the salt water can bind the screw to the housing, plus I think that there is some blue colored glue on the original screws. But I managed to get the back screw off (had to take a hack saw and create a channel for a flat head screwdriver to get soem torque on it as I stripped the tiny philips head!) I was afraid that I'd strip the housing, but I let WD-40 soak in for a few days. By the way, I replaced the screw with one from Home Depot - they're metric - M4 is the size.
Next I'm going to do mine now that I know it works!