I've been asked a couple of times where I typically buy my Lycra fetish gear. There's three different sources:
Danceries is a brand of ballet/dancing Lycra fashion, it can be found at numerous actual real-world as well as online shops. So you get the typical leotards, leggings or even single-piece suits. Everything has round-neck, and they don't really sell anything with a back zipper. No made-to-measure or other customizations like attached feet, gloves or zipper through crotch. But definitely "professional quality", i.e. seams so elastic that they never rip, fabric that can sustain a constant cycle of wearing, machine washing and drying for years.
AM-Sports is another supplier for literally hundreds of different items of skintight sportswear. They have any style/pattern you can imagine, accommodate special requests such as attached feet or crotch zippers and have the biggest choice of uni-color Lycra material. Alternatively they also have a selection of shiny effect Lycra, microfiber, velvet, PVC and (god beware) cotton materials in various colors. They don't do made-to-measure, but they offer both regular as well as slim/tall sizes. What I find very special about them are the so-called "vaulting fashion" styles, i.e. single-piece suits to be worn for vaulting. Obviously they're great even without any horses involved :)
Fets-Fash is a shop dedicated to the fetish community. Definitely no relation to sportswear here. Unfortunately this also means that their prices can be quite steep. Their specialty is customization with zippers, attached hood, feet, gloves, etc. They have some quite unique animal print and shiny effect Lycra materials. Made-to-measure service is excellent, but comes at a very big surcharge on the base price.