Obviously the challenge is to get float time. Pretty hard to do if you don't have a float plane. There are only a couple of places in the US where you can rent a float plane. You could try to get a job instructing on floats but it's pretty unlikely anyone would hire you with no time. Even if you were offered an job instructing on floats I would recommend turning it down. Compared to wheel planes, float planes give you very little time to correct a students mistake. You really have to know what your doing or bad things will happen in a hurry.

One option is to go to work for a company that operates both wheel and float planes. If you fly wheels for them for a year or two and prove that your not a complete idiot they may move you into the float flying. I saw an ad for a company in Kodiak like that a few years ago. The job ad was for flying a 206 on wheels with the "carrot on a stick" of moving up to floats after a season or two.

Your goal is not impossible, lots of people have done it. It just ain't easy.
Good luck