Rhystic Study, Mystic Remora, The One Ring, and Smothering Tithe are all pretty good options
Luckily, the Pokmon Trading Card Game has a wide variety of consistency cards that nearly every deck can use, and one of these phenomenal cards is Arven
The only comparable situation for Shadrix is giving the counters option to a creatureless control or combo deck, but thats also where the extra damage from those counters is most appealing
It's also worth considering adding budget energy staples like Aether Refinery, which duplicates energy you gain, and can dump all energy into one big creature
Compiling this list made me realize that the Legendary Birds don't have that many amazing modern cards
You will get one of the five Dracula series uncommons in slightly over half of the Box Topper boosters, and you will get one of the seven rares and five mythic rares in the remaining packs