Mademt33,
There are probably more reasons than one that you can't purchase rootlets in the Spring. One would be that the chance of losing them would be too costly since they would not be dormant and the chance of dying would be high, A 2nd reason would be that most rootlets are already sold by Spring. Some other guys may chime in about this.
I think you could plant rootlets now, just don't wait much longer. If you can get them planted before they try to grow a top it shouldn't be any different than planting them in the fall.