Ah - another ACL veteran! Congratulations on making it through. I had my auto-patella ACL operation back in April 2005, but I still remember the accident and months of PT like it was yesterday.
I liked Texasdivechick's suggestion that you seek PT-input about different kicking-styles, because new fins - especially force fins - can be pretty pricey. Thing is (I don't know what your current fins are like), is that the frog kick is easiest to perform with old-school short n' stumpy paddle fins. That wouldn't be very compatible with your poor knee right now, though. Still, some divers can perform alternate kicks wearing longer, slimmer blade fins or split fins (damned if I could, when I borrowed a pair of splits, so my hat is off to them!), and I've ready that that force fins do the job decently, too - I've never tried a pair of force fins myself. So, possibly a different kick (using either your existing fins, whatever they may be - certainly the cheapest option) and/or possibly new fins that will be kinder to your knee while flutter-kicking. Oh, and whether you keep your old fins or buy a new pair, the idea about spring-straps is really good - they'll be useful long after your knee is 100% healed.
Hang in there - and please feel free to contact me, too, if you have any queries about the ACL recovery process.