I started my own tack cleaning business last year. It's been slow and I only get customers at random times, but I think that's better than being overwhelmed with too much tack.
I posted my flyers at feed/tack/livestock supply stores, equine vets, the library, and at my riding barn. You could also try your grocery store, community center, and coffee shop. Once you get into the niche, you could use your profits to purchase a business card ad in a local horse newsletter/magazine.
What seemed to help me was that I got the most dirtiest tack I could find and cleaned it, taking before and after photos and putting them on my flyer. I did one western saddle and an english bridle to show that I do both types of tack.
I charge $10 an hour and have had no complaints, only compliments.
Remember to be safe! Always have an adult with you when meeting new customers and preferably have them meet you at your house/barn.