We do have a lot of bike shops here. Do you have a manufacturer in mind? I'll give you a list of shops I'm familiar with, but you might want to call and see what they have in stock before you visit--to narrow it down. What part of town will you be staying in?
Tour of Missouri is Sept 7-13th, if you have your choice of dates.
Big Shark is a nationally ranked shop near Forest Park. I didn't find anything there for me, but their website is bigshark.com. They host a lot of local races and a bike team.
We also frequent Ghisallo in Chesterfield Valley. They host rides and seem to cater to higher-end bikes and triathletes. ghisallosports.com
We purchased our Trek street bikes from Momentum Cycles (there is also a Trek store on Manchester Rd.) They have several locations--one is on the Katy Trail in St. Charles. They definitely rent. (If you have time to bike part of the Katy--Defiance, MO is a good start. The Defiance bike shop also rents.
Then there is always Touring Cyclist (my husband bought my first hybrid there, and they host several local/international organized rides).
Alpine Shop is a fun stop. They have everything outdoors--don't know that they have what you want in bicycles--but stop by anyway. They host rock climbing classes, kayak classes, etc.
We also have an REI at the Galleria. Not a great selection of bikes, but fun selection of accessories.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17