I provide upholstery catalogues to furniture showrooms free of cost - 3 catalogues quarterly. Showrooms place orders to sofa manufacturers, who buy fabric from me. How do I track sales generated by showrooms through these catalogues?

Basis limited knowledge of your business model here is what you can do. Start getting orders via email/message from your manufacturers (through website would be best). Request the sofa maker to quote the fabric number along with the order. If it is AB01 you know it is from showroom AB. If it is PQ20 you know it is from showroom PQ. Alternately, evaluate if these catalogue codes can be changed and made unique to each showroom next time you print a catalogue. The designs across all showrooms shall remain same, just that the design codes shall be unique to each showroom for easy identification. Eg showroom AB will have design numbers 1-20, same 20 designs will have design numbers 21-40 in showroom PQ, while showroom XY will have 41-60. So if you get an order for design number 45, you know that the order has come from showroom XY. Once you receive an order, record it in a log book and at the end of the month summarize your findings. You will be easily able to know which showrooms are generating sales for you. You may evaluate performance based commission scheme where the showroom gets X% of the orders they generate for you. This will also help you generate more sales. Hope this helps. You may message me and I can help you further.