yes there are, and you can buy them for about $5. Honestly though, if you have an old 3 pin fan just cut the wires cut the wires on your 4 pin fan and connect the 4 pin fan to the 3 pin adapter. It works like a charm (ive done this about a dozen times). Or if you want to spend money you are best off buying a cheapo fan off newegg for about $5 that has an adapter with it (many do).
Be VERY careful, though-- sometimes you can pull too much amperage through the fan, and ruin your motherboard because you sent too many amps through it.
If you've got a 4-pin fan, just buy a fan controller, and control the speed manually. They're not all that expensive, and you can usually control more than one fan with them. I've got one in a 5.25" bay that controls 4 fans from one spot.