My local Joann has this kind: http://www.joann.com/joann/catalog/productdetail.jsp?pageName=search&flag=true&PRODID=prd51106
but a different brand (Dritz). They were next to the Velcro dots, snap setters, and stuff like that.
There’s this style as well: http://www.buckleguy.com/categories/Rings-and-Loops/Double-Loop-Slider/
here are some in brass: http://www.dreamweaveryarns.com/catalog/slide-buckles-p-2421.html
here are some on etsy from the deceptively-named ‘sewing supplies’ shop which may have been mentioned elsewhere: http://www.etsy.com/listing/69360202/tri-bar-strap-adjuster-slide-34-inch-50?ref=sr_list_2&ga_includes%5B0%5D=tags&ga_search_type=supplies&ga_noautofacet=1&ga_search_query=purse+hardware&ga_page=1&ga_facet=supplies
here are some plastic ones: http://plasticcamstraps.com/
Would any of those work or is that not the kind of thing you had in mind? I made adjustable straps for the backpack with parachute clips since my kid likes playing with those, but I don’t see why it wouldn’t work for a messenger bag also. I suppose it depends on the look you’re going for? Oh, and you might want to try calling up Pacific Trimming — I am guessing from the fact that the only option they have under ‘buckles’ is ‘rhinestone’ that they might have other stock not listed on their website. If not, maybe they will know of another shop that can help?