Kate Middleton dressed her baby bump in stylish designers like Armani and Diane von Furstenberg. But most of us commoners can't afford to buy a whole new wardrobe for pregnancy or deck out the kids in high-fashion labels, especially when those kids will soon outgrow their clothes.

Thankfully for expectant moms and their brood of babies and kids, a growing number of online options make it easier to find affordable, yet fashionable, clothes. Here's a look at some rental websites and online consignment shops.

Renting Online

Dress rental company Rent the Runway pioneered a Netflix-like business model for renting designer dresses, in which the renter chooses a dress (or dresses) online and the item is shipped to her along with a prepaid envelope for mailing it back. Several websites including BellyBumpBoutique.com, BorrowforYourBump.com, MotherhoodCloset.com and RentMaternityWear.com now give similar options to expectant mothers.

[Read: Should You Rent Your Next Dress? ]

"When women are going to an event like a wedding, they're more apt to rent a dress because it's a little bit more formal and they only want to be seen wearing it once," says Krystal Stubbendeck, founder of Borrow For Your Bump, which launched last fall and is based in Omaha, Neb. "It's economical and you don't have to waste money on items that you can only wear for a couple of months."

The website offers more casual clothing for rental or purchase. Most items on Borrow for Your Bump cost between $20 and $50 to rent for 30 days and include free shipping for purchases of more than $75. For comparison, Belly Bump Boutique focuses more on formal dresses, most of which cost $39 to $79 to rent with a $12.95 shipping fee for every two dresses.

Some new moms save their maternity clothes in anticipation of more pregnancies, but if those pregnancies fall during a different time of year, they may need more seasonally appropriate clothes.

Stubbendeck says the rental concept appeals to women "who like to change up their wardrobe" and rent different outfit combinations each month. It also attracts women who change sizes sooner than they expected. "They were thinking they could get away with wearing their [regular] jeans [throughout their pregnancy] because a friend did, but every woman is different," Stubbendeck adds.

What about rips or stains? Borrow For Your Bump and similar sites typically dry clean formal wear between rentals. Stubbendeck says they also have a seamstress who can replace missing beads or make other repairs. Several of these websites offer damage protection for an extra $5 in case the renter damages an item while wearing it.

[Read: Beware of These Online Retail Pricing Strategies .]

Buying Gently Used Online

For new and expectant moms who prefer to buy rather than rent, plenty of websites sell gently used clothing. During her pregnancies, Christine Parizo, a mother of two in western Massachusetts, stretched her budget by buying maternity clothes on eBay. "It's not really worth it to buy maternity clothes new unless you're looking for something very specific, like you have a job where you have to dress a certain way," she says. "I knew that Old Navy's sizing was pretty accurate, so I basically searched 'Old Navy,' 'maternity' and my size. I looked carefully at the photos and asked questions."

Instead of buying individual items and paying to ship each one, she sought several pieces of clothing available in one shipment. In one case, she found five pairs of pants and five sweaters for about $40. "Don't be picky about lots," Parizo says. "They'll be one or two items you're not crazy about, but you'll find a way to make them work or donate them."

Plenty of baby and kids clothes are available inexpensively online, too. When Kelli Bhattacharjee, a Cincinnati mom and the blogger behind FreebieFindingMom.com, wanted a cute outfit for her son's one-year photos, she bought Ralph Lauren khaki pants and a sweater from Thredup.com, a website that sells clothing for women and kids. "I didn't want to spend $50 on a little outfit that he's probably going to wear one time, especially for kids when they go through clothes so quickly," she says.

[See: 10 Baby Products You Should Never Buy .]

While plenty of brick-and-mortar shops sell new or used clothes for babies and kids, Bhattacharjee prefers to shop online because of the convenience. "Consignment stores, in my opinion, can be like an outlet where you have to really search to find what you want," she says. "Here, you can set your filters to exactly what you want, and you'll know within 10 minutes if they have it. It's a very efficient way to shop."

Bhattacharjee has bought a few other items from thredUP and says she's been impressed each time. "The quality has been great, and it looks like new," she says. When her son outgrows his Ralph Lauren outfit, Bhattacharjee says she may try to sell it back to thredUP, since they buy items, too.

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