Be very, very careful with the "work" looks at places like Venus, Forever 21 and Victoria's Secret. Based on what I've seen, a lot of the stuff at those stores can very, very easily cross the line into work-inappropriate (even in a trendy office) because it's made with thin fabric or has plunging necklines or revealing cuts.
Gap and H and M will probably better options at your price point. Also, don't forget about places like Loehmann's, and check the sale racks at J.Crew and Banana Republic. Zara and Uniqlo are also worth checking out.
Remember to call your internship's office first and get a clear idea of the dress code. This should give you an idea of the type of items you should be looking for. Go through your current wardrobe and decide what items you already have that will work with the dress code, and what items you think you'll need. For example, you may be able to wear some of your current tops with a cute blazer over them.
If you are on a budget, it makes sense to focus on the basics that you are missing (like a jacket or cardigan, black pants, etc.) and use them to "dress up" the items you already have (as described above). If you want to add trendiness to your outfit, I would do what one poster suggested, which is get some inexpensive, trendy accessories (belts, necklaces, colorful flats, etc.) to add individuality and flair.
