Leasing horses can go both ways, good or bad depending on the needs, desires and morals of either party. If there are any miniature horse farms that show in your area you may find it helpful to show with them for a season, regardless of if you lease a particular horse from them or just spend time at their place working horses and going to shows. Many farms would love a second (or third or fourth!) handler at shows- for example if they want to show several horses in the same class. Its also great to have another set of hands around the farm for working horses, to take not only the time crunch off them, but it also allows several horses to be worked at once, thus improving their behavior and concentration at shows (if a horse is only ever worked alone at home cause there is only one person avaliable to train it then the show ring can be overwelming!).
I showed like this for many years as a kid, usually with a different barn and breed almost every summer. Everything from Draft horses, to driving ponies, to haflingers, and "run of the mill" english and western riding horses. Its an AWESOME way to gain experience inexpensively. I quite often was treated to free room and board at shows in exchange for grooming and handling horses at the grounds and between shows. I always declined any actual pay from any of the farms I "apprenticed" at but was offered several times. I never said no to good home cooking or a place to crash at a show though! LOL And I was often offered the exclusive use of one or several horses as my own "projects" to show and was usally welcome to keep them at the farm or take them home, during the show or off season. I usually picked a favorite or two to dote on, but worked with as many horses as I was allowed. I never took up the offer to take a horse or pony home, mostly because my facilities weren't set up for the type of horses I was offered!