
The first thing I must say - DO NOT use a prong/pinch or choke collar. If you have an aggressive dog, you don't want to use anything that will cause discomofrt or pain of any kind - it will be associated with other dogs when she's focused on them, and make her worse.
With that said, have you tried a harness/headcollar combination? I believe that is what Lindsay is describing, you use a double-ended training lead, with one end connected to the headcollar and the other to the harness.
My dobe girl is exactly hwo you describe yours on a walk - throws herself about on just a headcollar or just a harness, reacts strongly to other dogs. Unless we're following trails on one of our walks, I walk her on a headcollar/harness combo - I use a halti headcollar (would prefer a gentle leader but can't afford one) and a halti harness, with the lead attached to the ring on the back. It works wonders - took all of one walk for her to start walking at my pace, next to me. If she starts to pull I sort of jiggle the ends of the lead - tug gently on one, then the other, back and forth - and it stops her almost immediately. This, I should point out, is a dog with a 5 1/2 year history of VERY strong pulling, and being allowed to do whatever she wanted - the day I met her she actually pulled her owner into the road quite happily.
Re. the training; Lindsay's given you the good links I think. Look for someone who uses positive methods only, for the reason I mentioned above - any pain or punishment will make things worse. I certainly wouldn't take her to a training class - that would be overwhelming with the same result. I also wouldn't consider residential training, as you've no way to know exactly what methods they use.
One last thing, you mention no apparent reason for this behaviour - have you had her examined medically (incl. blood/thyroid testing)? PArt of Soli's aggression, both to other dogs and to me (resource guarding) stems from pain in her right hip, which I only realised recently. She's since had a tentative diagnosis of arthritis and a four-week course of chiropractic treatment, and the difference is fantastic - not only did I manage to give her the first bath she's ever had the other day, but she let a staff mix walk past a few days back only 3 feet away and didn't bat an eyelid!