dore_m
Based on my limited hands-on midi experience, as far as genuinely good resources, your best bet is to use those resources from the same brand/manufacturer as your DP , if they exist ….. or else run the gambit of trying CDs or free/downloadable pre-fab’d tracks.
As a big brand example, Yamaha offers such resources, for a per-item fee, which may still end up an unsatisfactory solution if you do not have a Yami model designed to utilize/integrate with Yami backing tracks not already built into your DP. 🙁.
A midi reality is, yes, its a standard, but when it comes to midi uses such as play-along with backing tracks …… tracks from a given source A, B, C etc, that plays/sounds the same or close-to-same on any big brand x, y, z, etc device - that don’t also require a lot of “articulations tweaking” - are not.