Cover is everywhere
But also remember that a fair number of D&D combats occur in forests, grassy fields, city streets, mountains, large caves, or ruins--all places with plenty of hiding places (especially for Small creatures). Very few places (a dry lakebed, the middle of a huge audience chamber, an arena, some parts of deserts, open tundra) are without cover.
And if there are a large number of combatants and you're a Small rogue, even those places have cover--you can hide behind your friends, behind bodies, behind dropped shields and other battle debris.
The thing is, hiding in combat is a two-step (as in, two-turn) process unless you use bluff or have some other fancy trick like Spring Attack or Shot on the Run.
1) Move out of sight and into cover
2) Hide
The key point is that you have to move. So if you have buddies standing toe-to-toe in combat and you want to hide, you're taking yourself out of the fight for at least 3 turns (moving away, hiding, moving back while hidden).
-z