HS-55's are made by Hitec and are available from many online retailers and a majority of hobby shops I've been in sell them. The only complicated thing about changing the servo's out is keeping the wiring straight. You can re-use the stock servo arm (it's a very tight fit on the HS-55) or you can just use the ones that come with the servo. I haven't tried the servo arms that come with the Hitec servo's, so I don't know how tightly the control rods fit in the holes on the Hitec servo arms. If the holes in the Hitec servo arms allow the control rods to wiggle even the slightest, I wouldn't use them.

I think the heli dropping when cyclic changes are made is addressed by a few people in another thread about the Axe.

EDIT:
I did get the one that stripped replaced under warranty, but I still put the 3 HS-55's in... I think the servos that came with the helicopter will be fine in a parkflyer... they'll probably end up in the f-18 my son wants me to build before the blue angels come to town this summer.