You rarely need a commissioned study to size a market. You need a number you can defend, built from sources someone else can check, with the assumptions visible. That is achievable in a couple of weeks.
Start from registers, not reports
Business registry data tells you how many entities operate in a sector, where they are, and in many cases gives you filed financial information. That is a real foundation: a count of competitors and an order of magnitude for their revenue.
Statistical office data adds structure — household expenditure by category, sector output, employment. It is less current than you would like and more reliable than anything you will estimate yourself.
Triangulate with trade data
For anything imported, customs and trade statistics are the most underused source available. Import volumes by product category give you a ceiling on domestic consumption that is very hard to argue with, and tracking it over several years shows you the trend without any modelling at all.
Where a market is served largely by imports, this single source can get you most of the way.
Twenty good interviews
Then talk to people, with a structure. Not a survey — twenty conversations with distributors, retailers, installers or buyers, asking concrete questions: how much do you sell in a month, what does it cost you, who else supplies you, what has changed in two years.
Concrete questions produce usable answers. Opinion questions produce noise. Two or three distributors who describe the same pattern independently are worth more than a broad survey with a low response rate.
Show your working
Build the estimate two ways — top-down from consumption data, bottom-up from units and prices — and see how far apart they land. If they agree within a reasonable range, you have something defensible. If they do not, the gap is telling you which assumption to investigate.
Write the assumptions on the same page as the number. A figure whose derivation is visible can be challenged and corrected; one that arrives without provenance simply gets ignored the first time it is questioned.