Ankle weights may only be a solution for beginner divers. The fact that your feet may be floating is not a real problem, the problem is when your whole leg floats as this causes a feet up head down profile which is bad. (Especially when diving in a dry suit)
To get your feet down get yourself a pair of heavier (less buoyant) fins (jet fins tend to be heavier and favored by tec divers), you could also use thinner wet suit boots. Another option that helps is to practice leg muscles and use them to keep legs down, the larger and heavier the leg muscles the less chance of floating, also look at moving weight belt lower on your body. It is all about in water TRIM.
If you look at technical divers you will see that most of us have our feet up on purpose. Especially when penetrating areas with silt deposits as we do not want to kick up the silt. (bend knees and have feet higher than legs)
As I stated before floating feet are only really an issue when your feet float up and your head down and this you most likely could correct by moving the weight belt lower on your body and moving your cylinder down as well. It is amazing what you can do to your profile by distributing the weight differently across your body.
In a dry suit you could use straps to help keep air from going in your legs and boots.
If you still have problems seek a divemaster or instructor, especially one that has technical diving experience, and ask them to help you with this. If they say buy ankle weights you may want to look for someone else.
Remember it will take you up to 20 dives to get your weighting and trim correct. Please practice improving your diving position (trim) on each dive until it becomes natural and comfortable. PRACTICE DOES NOT MAKE PERFECT! Practice forms habit. PERFECT PRACTICE FORMS PERFECT HABIT!
In the words of Dory: "Just keep swimming"