If you really can’t do chemicals, it’s going to be harder.  First off, a Glue based flee trap will help you monitor progress and warn of flees returning.  They do not capture enough flees to make a serious dent in population though.  Pick a spot close to he bunnies, where they can’t actually get them selves stuck to it.

Next, you need a fine toothed flea comb, and a bowl of slightly soapy water, another of fresh water, and a towel to dry the comb.  Use the comb to comb out any flea on your bunny.  drop the fleas into the soapy water and they will drown.  If you drop them in water without soap to break the surface tension they can swim, the soap is necessary to kill them.  After drowning each flea, rinse the soap off the comb in the plain water, and dry it off.

They also make zapping flea combs, I have never tried them, but parents I know swear by the lice version for children, never actually tried those either.

To a large degree, the fleas live in the environment as much as they do n your bunny.  Borax and diatomaceous earth kill fleas through desiccation (removing water) instead of toxins.  It is important to treat the environment.  Work the descant down into the carpets, rugs and upholstery of your house.  Use a stiff bristled broom or brush to really work it deep into carpet fibers.  Make sure you are getting into all the crevices and cracks in your house, such as between floor boards.  Leave it down for a few minutes, and then vacuum like mad.  Repeat this process.  Do the whole house, not just places the bunnies go.  Change the vacuum bag when you’re done, and take to the outside garbage.

After vacuuming, leave more desiccant any where the bunnies (or any small children) can’t actually lick it, such as around the edge of rooms bunnies aren’t allowed in, underneath carpets, etc.  If you live in a warm climate, spread diatomaceous earth in your yard.

Watch out for less obvious flea hiding spots, like laundry.

Don’t view this as a one time thing and expect the fleas to be gone.  You will need to repeat this activity daily or every-other-day for several weeks.  Even if you see immediate results, you need to repeat it.  All you need is a single pregnant flea, or two eggs to survive to have the fleas bounce right back.

Make sure you are treating the environment and bunnies simultaneously.  If you comb out the bunnies, then put them back into their uncleaned condo, the fleas will hide on the bunnies while you are cleaning.  Put the bunnies somewhere flea free while you clean the rest of the house.  The bathroom would work well, since it can be cleaned very easily.