Well, here goes nothing:

Ever tried searching for a card that ou can never get? Have no fear! Below,
I'll list some Uncommon and even Common cards that can just about do what a
more hard-to-find card can do.

Alakazam: I have just about dozens of Abras and Kadabras, and I've no
Alakazam. However, since the Fossil pack is already out, your next best thing
is Slowbro! The more in your deck, the merrier. When your Active Pokémon is
in distress, switch 50 damage to a Slowbro and whatever you have anywhere
else onto the others. As long as you keep it on the bench with no energy, you
can Poké Center them and get up to 200 damage healed on a single turn! Just
put 4 PokéCenters in...

Charzard: Yes, there is an uncommon who can stand up to Charizard's
level...and that Pokémon is none other than Arcanine! Charizard can do 100
damage, discarding two Fire energy. Arcanine might need an extra turn to do
it, but you can do what Charizard can do, minus one energy. its Take Down
attack does 80 damage. These numbers are much more useful than 100, which is
either overkill now or overkill next turn.

Poliwrath/Dragonair: Everyone knows what Poliwrath and Dragonair both have in
common. They both remove Energy. golduck's Hyper Beam may not do as much
damage as Whirlpool, but for less energy than either of the others' attacks,
Golduck is a good choice when you can't get those Rares. Besides, Poliwath is
a Stage 2, which take a while to unleash.

Hitmonchan: Okay, okay, this Pokémon will take a whole lot longer to charge
up than Hitmonchan, but who can refuse to say that Rhydon isn't all its
worth? 100 HP, for three Energy, do a mediocre 30 damage, and add one more
Energy, and you've got a Lure that can do 50 damage! While Hitmonchan can do
50 for less without damaging itself, Rhydon makes up for it with its
incredible HP and its infinite Gust of Winds.

Base Magneton: This Pokémon, although is a different type, is yet another
uncommon with similar attacks: Golem. I don't know about Fossil Magneton, but
Golem can do 60 damage in place of Magneton's puny 30 (?) damage attack. They
both Self-Destruct, but Golems Explosion directs it all to one Pokémon. So
when you're going down, wouldn't you rather kill off the Defending Pokémon
(with the exception of Charizard) than do just 60 damage? Oh yeah! Fossil
Magneton can do 20...

Venomoth: Here's a Rare that can be beaten by a common called Koffing.
Venomoth has the strange ability to change its type, but...wow. You do 20
damage instead of 10. For equal Energy requirements, damage, and effects,
Venom Powder and Foul Gas are also very much different. One thing is that
Koffing has a 100% chance of affecting the opponent in a negative way, but
Venomoth can only do it 50% of the time. Of course, Fossil Magmar could do it
too if you're making a Fire deck.

Nidoking: Nidoking has two attacks, each that costs 3 energy. The max they
both can do is 40. For three Energy, there is another Pokémon who, for three
Energy, can do 60 max. That Pokémon is Exeggutor. I don't quite know what
Teleport is for except for stalling with someone like Lickitung or Onix. Big
Eggsplosion is lie Water gun, except better: You have no limit as to how much
Energy you can slide into Exeggutor. However, since a coin needs to be
flipped, there is an average of 10 damage per Energy. Of course, Thrash does
30 damage as well and Toxic is useless against anyone with no retreat cost.

There's more Uncommons and commons that can be used in place of Rares. I
can't find a match for these guys, but I can tell that there are Rares who do
worse that they do:

Dewgong: 50 damage or Paralysis, you choose
Farfetch'd: 30 damage on first turn if you're lucky
Haunter: Subliminally do 50 damage during sleep for low Energy cost
Jynx: Can you say "Meditate"?
Kadabra: Need one more Energy, but can do 50 without sleep
Machoke: Can kill any Electric Pokémon in one hit, or otherwise 60
Nidorino: Same as Kadabra except on further energy is needed. DCE
Poliwhirl: Can beat Charizard in one hit. Amnesia is great as a stall attack
Raticate: A valuable Pokémon in conjunction with Mr. Mime and/or Jynx
Metapod: Not bad for a Common. Can keep the enemy from damaging for a while
Starmie: Recover says it all. Oh, and Star Freeze ain't too bad either.
Tangela: Not too much damage done, but can Paralyze AND Poison...
Butterfree: The one Pokémon who can either GOW or heal itself
Dodrio: Save your Energy! Retreat for less!
Tauros: A Basic Pokémon who can do 70 damage without side effects (50% of
time)
Weepinbell: Poison the enemy, attach another Energy and do 30 damage plus 10
Exeggcute: Can do what Jigglypuff can do and can heal itself too!
Jigglypuff: Put 'em to sleep as soon as you get it out and do 20 damage next
Golbat: Healing powers and an all-out offensive attack. Very versatile
Omastar: Can damage like Poliwhirl for less
Sandslash: 60 damage max
Seadra: If you're extremely lucky, you won't get damaged for quite a while
Tentacruel: Can confuse AND poison too
Geodude: Can do infinite damage
Tentacool: Cowardice proves to have its rewards greater than bravery