First store where you bought your gaming supplies. Does it still exist?
It was quite a while after we started out with our second-hand Moldvay Basic set before we felt any need to acquire more D&D products, but eventually that need did arise. Specifically, we really, really needed the Expert rules so we knew where to go with these characters who were outstripping the limits of Basic. I had simply extrapolated the XP charts in a linear fashion as a stopgap measure, but we needed the real tables. We needed the higher-level spells, and we needed some new monsters and treasures, dammit!
The only place we could find at the time that carried D&D stuff was a little toy and hobby store in the Clackamas Town Center mall in Portland, about 45 minutes away when traffic wasn't bad. I was still a few years away from my driver's license, so the only way I could get there was to beg a ride. They had probably the best selection of board games I'd ever seen, though I wasn't particularly interested in those. They also had an entire shelf full of classic D&D rule sets, modules, and supplements. I had never realized just how many things there were to add to this game, but once I knew, I wanted them all! It was always agony deciding what to get, because my meager budget wouldn't allow more than one or two at a time. I only went there maybe three or four times, and can't remember the name of the place to save my life.
We stopped going there after the Waldenbooks in the mall here in Vancouver, Washington started stocking D&D products, and I haven't been back that way since, but I'd be shocked if it were still there.