It goes without saying that the very best place to buy houseplants is a nursery or garden center, where there are people knowledgeable about the plants and their care. There are also likely to be better selections and healthier plants. That said, there's nothing wrong with buying plants at the other places mentioned (who hasn't fallen in love with a plant they found at the grocery store?) as long as you follow a few basic rules:
First, educate yourself before you buy. Nothing is more disappointing than falling in love with a gorgeous plant only to have it die because it didn't like the conditions it was in. It's crucial to know basic things like the kind of light your home gets. For instance, a plant like a Croton will be miserable in a corner by a north facing window. Crotons love bright sunlight-in fact their brilliant colors will fade without it. It's also important to find out what kind of care a plant needs-for example, a Burros Tail won't do well in a house full of kids, as its leaves are extremely fragile and fall off at the slightest touch. Speaking of kids, if you have them, or pets, please be sure to check and see if the plant you're considering is safe for them. Some houseplants, though beautiful, can also be poisonous. A dieffenbachia, for instance, will make the throat swell and cause loss of voice if its leaves are eaten. (Perhaps this is why its nicknames are "Dumb Cane" and "Mother in Law Plant"!). Get to know your store's schedule too. Getting there the day their shipment of plants arrives gives you a better chance of getting both a good selection and a healthy plant.
Look over any plant you're considering very carefully. Plants sold in home centers, grocery stores, and the like, are often cared for by people with little or no training in plant care. That's why plants are often found either swimming in their pots or bone dry. Sometimes you'll even find plants in each state right next to each other. This is why it's crucial to examine them closely. Look at the leaves and under them, and also at the area where the leaves join the stem. These are prime areas for insects to hang out. Also look for mushy stems, brown spots, or any signs of disease or infestation. If the plant is covered in a sticky substance, put it down. While it's normal to find a broken leaf or stem (houseplants are shipped in boxes on large pallets and aren't always handled with care) avoid any that look like they are dying. Most plant lovers find themselves with the urge to "rescue" a plant, but it's really not worth the risk to your plants at home. It's also important to examine the soil. It shouldn't have any mold or slime on it. Don't be afraid to sniff it. A fishy smell could mean the plant was treated with a fish emulsion fertilizer, but it could also mean it's infected with a bacteria or fungus. Don't chance it. Finally, take the plant out of its pot and look at the roots. Are they firm and healthy or black and mushy? Rotted roots are a sign of overwatering.
Rule #4: Quarantine! When bringing a new plant home, keep it away from all other plants for at least a week. During this time do regular checks for any signs of insects or disease. If after a week or two, the plant seems healthy and happy, it can then be moved it to its new home. Let it acclimate for about a month before repotting, if it's necessary.
By following these simple rules you'll be sure to have happy, healthy houseplants no matter where you buy them!