Perhaps I can help in regards of the english textile industry and american ready-wear market during the 18th and 19th centuries. To understand the existence of the ready-wear we must first analyse the technologies conditions that made them possible in the first place. The textile industry was the the flagship of the british industrial revolution, which began around the year 1780 (I'm going with Hobsbawn definition, but you may find someplaces stating it began in 1760). Around that time the british empire already had a strong grip on the global market, importing large quantities of cotton from India, whose market they conquered through political and military force, and from the United States, its former colony. This garanteed a vast and steady flow of a raw material that was cheaper and easier to work with than wool or linen, and was one of the main factors that made the industrial revolution happen in England rather than other countries.

That's not to say that the technological advancements were not important, because they very much were. The most famous ones are problably Richard's Arkwright Spinning Jenny, patented in 1761 and could draw threads from multiple spindles, as opposed to the normal spinners that could only draw one; In 1769, Arkwright patented the water-frame, and different kind of spinning machine that was more versatile, as it could work the fiber in multiple ways, but more efficient too (the water-frame if very technical and hard to explain without in-depth knowledge of it. I'll put a reference to an article explaining it at the end of the post so you can check it out); In 1779, Samuel Crompton merged the designs of the Jenny and the water-frame to create what was called the Spinning Mule, and it was able to make finer, smoothier and more consistent yarn from multiple spindles. In fact, the mule revolutionized the industry in a way that in the cotton mills just around Manchester, 4,206,670 spindles were being worked on mules, against 156,000 on Jenny's and 310,000 on water frames. By the end of the 18th century Britain not only had the materials, but the capacity to work with them, and during the 19th was the major exporter of cotton fabrics in the world.

That being said, untill the mid 1800's sewing took an awful lot of time, and it was mainly done in a domestic system with manual labour. While sewing machines appeared in the early decades of the 19th century, they would only gain traction around 1845, the Elias Howe creation, and later in 1861 with Isaac Singer's patent, based on Howe's machine. Now, these small shops already existed both in the US and and Britain prior to this invention, and at least in the US were marketed towards sailors and located around wharf areas, supplied by tailors and seamstress. Still, a good deal o people bought clothes on second hand shops.

The development of the sewing machines opened the way for the expansion of ready-wear shops, especially in cities such as New York, Baltimore and Philadelphia. So we can have an idea, an experiment done by manufecturer Wheeler and Wilson published in 1861 stated that the avarage time to stich a summer pant went from around three hours to around fourty minutes; a calico dress from six and a half hours to one hour. Not only this, but the sewing machine making some pieces of clothing quicker to make also made the whole process cheaper, therefore increasing its popularity, such was the case of cloaks, for example. It's worth to note that the technology also made shoemaking easier and cheaper, and was beneficial not only to the general populace, but also to the Army.

The first machines weren't cheap though, and it was not uncommon for companies to lease machines to tailors and seamstress, or even selling on credit (the buyer paid a little sum upfront and then paid the rest over a set number of months). It was only with time that the prices went down and a proper market was formed, with bigger shops and decentralized management, that the domestic system began to shift to a more industrial one, a little closer to what we know today. Keep in mind that, by that time, we still didn't have a fashion industry and consumerism was very different during the 19th century, and most of the clothes bought by the working class, and even the better paid ones, were from second hand shops.

Sources:

Eric J. Hobsbawn (2011) The Age of Revolution: 1789 - 1848 (Portuguese Edition)

Diana de Merly (1986) Working Dress: A History of Occupational Clothing. Holmes & Meier

Barbara Hahn (2016) Spinning through the History of Technology: A Methodological Note, Textile History

Cooper, G. R., The Sewing Machine: Its Invention and Development. 2nd ed., revised and expanded. Washington, DC: Smithsonian Institution Press, 1976.

Edit: Grammar here and there.