Actually, there is a fair amount available now, but it does take some hunting skills and patience! The biggest gaps are in UK equipment, I suspect largely because the home British market is so entrenched in OO scale rather than true HO the plastic manufacturers ignore it. Though there are small resin casters.
When I started collecting for gaming in the mid-70s, the scale choice was either 1/285 or being willing to mix and match 1/87, 1/76, 1/72. I went with 1/87 as much as I could and especially tanks, and filled in with 1/76 as needed, and 1/72 for some small vehicles. In the last few years, I've been able to phase out the larger kit and fill in with new 1/87 with trades and purchases. The joys of the internet! And lots of Roco still pops up on EBay. Fidelis has been great for the most brands in one place:
http://www.fidelismodels.com/mm5/merchant.mvc I'm still catching up on painting and basing the replacements, eventually I'll post battalion pictures when I have a quorum ready for their photo ops.
Trident and Preiser have a lot of German stuff (and fortunately some discount retailers!), as does SDV from Czech (but very short run plastics, I've grabbed them as they came out). Some of the Boley die cast stuff is true to scale, their M5 Stuarts, Panthers, and Jagdpanthers are nice.
Heiser has the widest range of resin kits in a wide variety of nationalities, and an armload of different Sherman models in plastic (some with resin bits) which are way better than the old Roco Sherman (that one didn't match any actual version of the tank!). Heiser's are hard to get hold of though since it's a one-person basement operation -- and best done by direct order and wait for him to cast it, but I've gotten a fair amount through a variety of retailers. I'll be placing an order with him as soon as he finishes his new price list.
Mo