I will probably be brow beaten for admiting that I use a flexi lead just when walking Vash.
I don't have a fenced in yard, so the most freedom Vash gets outside of puppy day-camp is with a long lead. Initially I was using a 20ft training lead but Vash walked over it, peed on it, drug it behind him on the side walks, it got pretty unsanitary. And after a few washes it wasn't really safe to take him out on the leash because it was frayed and not even duct tape could save it.
After my 3rd training lead I just decided to go with a flexi lead. This has had good and bad side effects. The good is that the flexi lead doesn't get as nasty and frayed as the training lead. The bad is that you do not have nearly as much control over your pup. I am constantly adjusting the length of the flexi on the fly and when people walk by me on the side walk I often have to reel Vash in like I'm a fisherman. The other downside is that Vash seems to pull more now than he ever did on the standard leash.
The most irritating issue is that if you have to pull your pup back on his leash for his saftey while the lead is extended you have to grab the rope and yoink on it with your hand, this is rather painful for you especially if your dog is on the other end pulling.
And it tends to get stuck in places that normal leads would not. The other day I was walking Vash and he decided to sniff a fire hydrant, somehow the lead got caught up in the chain link around the hydrant and I spent a few min having to get it out.
That said, make an informed decision about what you believe is best for you and your pup. By now you've read all the warnings, and hopefully my post will give you more of a hands on type of perspective of what using one is like. If you still want a flexi lead then just about any will do, I think Costco has a little kit that has a flexi leash, the flashlight attachment and a harness if i recall for about $25.
I would simply caution you to be cautious while rollerblading with your pup on one. All your pup needs to do is get himself wrapped around a pole and next thing you know you'll be on your bottom the flexi lead will be retracting, and your pup will be running away from the retracting flexi lead as it heads straight for him making a heck of a racket.