The one on I-70 near the Zumbhel Rd exit is Mark Twain hobby shop. Only been there once as I have just started a layout. Were very helpful and seemed to have a good selection. This is on the north service road just east of the exit and is located in St. Charles. I found their website when looking at KATO’s dealers on their site.
I used to have St. Louis hobby and pet stores memorized. I know some have gone out of business since I was there, but as others have stated there is the train hause - I believe that is in Warrenton. There was in St. Charles proper down on the river walk. Then Schafer’s Hobbies on 116xx Gravios. Dasho’s, Checkered Flag (in Ballwin), and Des Pere’s Hobbies all on Manchester Road fairly close to I-270.
There are a lot of good hobby shops in STL. Hobby Station in Kirkwood is good, All of the previously mention Schaffers, Mark Twain, ect. all have very good points. It kinda depends on your “selling Point” as to which one is good. For “n” scale, Schaffers and Checkered Flag both have decent selections. Hobby Station has the “old time” hobby shop feel to it. Is there some item in particular that you are looking for?