Try asking the lfs if they will take your shark. If not advertise him in the buy and sell section. You may not actually get any money for him, but at least you are keeping your other fish safe.
One of the hardest things when starting in the hobby is training your eye to accept the relative bareness of a healthily stocked tank. This is because we see the tanks in the lfs which are choc-a-bloc with fish.
The first thing to remember is that shops have super-massive filtration systems that we just can't emulate in the home, so the stocking levels in a shop would kill the fish in a home aquarium in no time.
Secondly, the fish are only in there for a short transit period when they are still too shellshocked for aggression problems etc to become apparent. I like to think of fish in a shop as passengers on a packed commuter train. It doesn't kill them, they cope with the conditions on the Woking train with reasonable cheerfulness, but it would be mistaken to believe that human beings could cope with conditions like that for weeks on end without pandemonium breaking loose!
Thirdly, the fish sold in the shops are almost invariably juveniles, so they don't have to cope with problems that arise later in life, whether because of growth or sexual maturity=aggression.
And fourthly- a lot of fish do actually die in fish shops, many more than we would consider acceptable at home.
If you are not in a position to get a bigger tank (and lots of us aren't), then there are ways of making the present tank with its current inhabitants more interesting.
First of all, decorate it. Your guppy may appreciate plants (whether real or artificial). Your cories will certainly appreciate some sort of hiding place, amybe a cave, or a piece of bogwood.
Secondly, read up on the species you have. Makes them a lot more interesting if you understand about their behaviour.
Thirdly, feed them a good and varied diet (but don't overfeed, particularly not while the tank is cycling).
But most important, work on maintaining your water stats!
It is a little hard on your only guppy, as they shouldn't really be alone, it's not good for them. But then you can't add more fish at the moment, and as male guppies can be a bit funny with each other in an all-boys group, you would have to add 4 of them, which you won't have room for. If this guppy does pass on, I would wait for several months, until the tank stats are absolutely stable, and then add a small group of some small fish like endlers or glowlight tetras or dwarf rasboras (if you can get them).