As it turned out, I went to Lukas Liquor. Wow, best selection of rum I've ever seen (granted, compared to provincial monopolies in Ontario and Quebec). There was a lot I wanted to buy, and if I had more room in my car (and more $$) I would have walked out of there with at least 20 bottles. I ended up with 5. A Plantation Rum from Jamaica, I can't remember which specifically, but it was on sale and I drank it during my stay in St. Louis (both neat and with coke). I liked it. The four bottles I brought back with me were the 18 yr Flor de Cana, (which I haven't seen in Ontario); St. James Hors Age, (only the basic white and brown are available in Quebec); Rhum JM, a 50% white rhum from Martinique; and Chauffe Coeur.
I haven't opened any of them yet, I want to finish the 12 yr Flor I have open in my cabinet before I do, but I'm thinking I'm gonna wish I had a Lukas Liquor closer than an 18 hour drive from me
There were a lot of rums I saw which I wanted to try but I focused on rum Agricole from Martinique (though I understand the Chauffe Coeur may not actually be from Martinique). They had no rums from Guadeloupe sadly, but several interesting looking ones from Venezuela and they had the 15 yr Barbancourt which I regret not getting, I've only tried the 4 yr which is available in Ontario.
Another thing which made it so tempting, alcohol prices in Missouri are roughly HALF the price of Ontario
I was real fortunate at the border too, I did declare the rum (and the wine and vodka I brought back over the border) and the border agent waved me through!