Vintage sundresses or sundresses that look vintage are always a good choice, A low-heel wedge. Fitted jeans or capris look nice for casual wear with a fitted top. I like a cat eye with nude eyeshadow, a little blush and red lipstick. A couple of wide bangles or bakelite bracelets and you have a summer casual look.
I found a few items on Modcloth.com they are a little cheaper by about in price. The only thing is the quality. I accidentally brought the The Erin Wiggle on PUG then went to Modcloth and purchased the same style dress over there just a differnt color. I paid less at Modcloth but when the dress came is was cheaper material and the zipper was higher up my back.
I ordered a casual dress that I'm going to wear today from there. Very simple and cute but same issue with the zipper and I can guarantee I won't get very many wears out of it. Maybe 6 or 7 wears before I'll have to replace it. It was pretty cheap though only $47. There's a few sites out there but you are still going to have to come out a little bit in the pocket.
I'm pretty much and online shopper for vintage inspired, so that you found a few places to walk into is great.
Ebay, Etsy. I am lucky that Portland has a ton of vintage stores so I can find loads of great scarves in town too.
Try ebay or etsy.
catslikeus.com - check their accessories section.
I tend to find mine in antique stores. There's one antique store I go to that they have all their scarves in a box and you just rummage through them till you find one you want. They're normally pretty cheap too. As for clothes, Forever 21 actually has stuff that will fit your casual vintage needs at an inexpensive price. They carry many cute blouses that look adorable with high waisted jeans, pencil skirts, or circle skirts.
Second ebay. I've scored some wonderful lots of scarves for just a few bucks.
For hair I might pull my hair back into a ponytail then add a hair flower (or two!), or if I'm feeling extra motivated, I'll do my hair up in pin curls the night before (I figured out that all it takes to get some nice curl is about 12-20 large pin curls with my hair, which is as straight as you can get and quite thick). For makeup, I wear tons of mascara. I think the key in getting the right shape for eyelashes is to find an applicator brush that works for you, then use it with any mascara you like (I get the $3 Rimmel tube). I usually coat the lashes, brush them out with an old applicator, then apply more mascara before the first coat is completely dry. It doesn't take long, but if you want your lashes to pop, it's the best way I've found to do it. I also like tinted lip balm or lip stain. It's a little less 'dressed up' than lipstick, and easier to maintain. I never wear foundation, but I do wear some cream blush. For clothing, I have a lot of PUG now, but I do mix it in with mall store pieces since as you mention, separates can add up. The Doris skirt works with a lot of my mall tops. And I wear my Joanie tops with Old Navy jeans all the time. It takes a bit of searching, but if you stick with basic separates (not of-the-moment fads) and mix them in, the look is still on the retro side and easy. For jewelry, a simple string of pearls and pearl post earrings are the most I wear now, but it's a classic look, and affordable since synthetic pearls and freshwater pearls are easy to find and inexpensive. Check on etsy for some vintage brooches and jewelry that you can mix in with your wardrobe. So few people wear brooches anymore that they stand out when someone does (plus, there are tons of them available for sale so you can find some great ones for not too much money).
tarte's lip stain in lust is a good one. it's not super long-wearing like a lot of lip stains, but it's easy to apply and lasts pretty well if you blot between layers. i really like it for casual wear because you can build the color so it's anywhere from a light raspberry stain to a true red. lip ink make great, majorly long wearing lip stains- i actually first heard about them from dita von teese on twitter, and she knows what's what for sure. :) and i've head that revlon's are really good but i haven't tried it myself yet.