I'm making a draconic heritage (Races of the Dragon) sorcerer and rolled a d42 (on roll20) and ended up with a shadow dragon heritage. I want to get the Draconic Power feat but having a hard time finding enough spells with the [energy drain] descriptor to justify it: how or where can I find those? I know of the spells in Player’s Handbook but I don't have many books so my knowledge base is quite limited.
There is no "energy drain" descriptor or subschool
Draconic Power (shadow dragon) is nonfunctional as-written.
If you look at the enervation and energy drain spells, you'll see they have no descriptors or subschools. Likewise, in my searches for similar things, I have found no indication that an "energy drain" descriptor or subschool has ever existed in 3.0 or 3.5.
Underneath the table on page 103 of Races of the Dragon, it states that some options for Draconic Heritage do not have spell descriptors—instead, they have an associated effect. For Draconic Heritage's bonus on saving throws, that's fine, but for Draconic Power, the feat doesn't function by the rules (it calls out particular the save bonus for Heritage, not all the relevant feats). Thus you're going to need to enter the realm of DM rulings and houserules for this.
My recommendation would be to talk to your DM about it. Energy draining effects are somewhat rare, and in many cases, don't prompt meaningful saves at all (energy drain, for example, prompts a save 24 hours later. Most enemies will not live that long, thanks to how this game handles encounters. Enervation, on the other hand, has no save at all).
One possibility could be to ask for it to apply to all negative energy and negative level-causing effects (either ones called out in a descriptor, or ones called out in the text), as well as possibly spells affected by the Fell Drain metamagic feat. This seems comparable (though somewhat niche), to the potential boosting of all damaging spells of a useful/focused-on element (most dragons), though much weaker than some of the similarly-unique dragon choices, like chaos or deep dragons.
No spells have an [Energy Drain] descriptor
Not all of the 'energy type' entries listed are actually energy types or descriptors. The text in Races of the Dragon regarding the special GM-permission-only list the shadow dragon heritage is on states in part:
Many of the draconic bloodlines listed on the table don't grant a bonus on saves against effects of a particular energy type. Instead, you gain a bonus on saves against spells and abilities associated with the effect (such as force for the Tarterian dragon) or subschool (such as compulsion for the chaos dragon) that matches the entry on the table, as well as the normal bonus provided by the feat against magic sleep and paralysis effects.
Energy Drain isn't a descriptor or subschool, it's a kind of effect. Energy drain effects are those that use the Universal Monster Ability Energy Drain (Su) or specifically call themselves out as an energy drain effect. Probably the 9th level spell energy drain also counts, even though it doesn't technically invoke the ability. Basically your DM has to decide what is an energy drain effect and what isn't, but they already needed to do that to adjudicate the ubiquitous 4th level Cleric spell death ward so it isn't really any extra work.