...did someone say that they're looking for 18" 870 Express ideas?:
The accessories are out there, it's just a matter of deciding what you want (which may not necessarily be limited to what you actually
need) and how much money you're willing to spend.
In many cases, an 'accessory' can be sourced from more than one producer over a wide range of prices ...side saddles are a good example. Although TacStar and Mesa Tactical market products that are similar in appearance and function, the difference in quality of construction is significant (as is the difference in price).
Also - if you're not in a rush - many of the best accessories that aren't yet available from Canadian retailers can be sourced directly from their U.S. manufacturers. Knoxx, XS, Hogue and Mesa Tactical are but three U.S. producers of some fine shotgun accessories, with whom you'll have no problem placing and importing an order into Canada.
Depending on how much you want to invest into this, you might also want to consider having the gun refinished. The factory 870 Express finish is 'utilitarian' (to put it politely) and having it redone can make a big difference in the gun's aesthetics. The 870 Express in the above picture had its original factory finish removed by media blasting, then the entire gun - including any metal accessories - was refinished in baked-on satin black Teflon.
The 18" 870 Express is a terrific platform on which to build an accessorized 'project gun'.