Fellow first year law student here who just put his clothes in the laundry for the first time in a month. Hopefully you pulled some excellent first semester grades to justify the slovenliness.
I'd probably go with an H&M sportcoat (Target if you don't have and H&M in your area). In terms of getting a better sportcoat, you might want to check out Anderson Little.http://andersonlittle.com/
My roommate just picked up one of their Navy blazers and it looks great. Pair that with J Crew Chinos (their slimmer cuts are a lot more flattering than their more relaxed fits) and you'll be off to a good start. For shirts, I actually really like H&Ms slim fit line. Supposedly they don't have the best construction, but I haven't had a problem with them yet and they look great. I also like Charles Tyrwhitt for spread collar shirts. They almost always have some great sales.
For ties, look on the buying and selling board here. You can usually get some great labels for rock bottom prices.
Suits are a bit trickier since there's a wide array of quality at different prices and if you aren't careful you'll drop a lot of money and have a crappy product. I'm surprised that you got through your first semester of 1L without a suit. Regardless you're going to have to have one for your summer work and for OCI. I have a Banana Republic suit that has served me well enough, but the fit and construction just aren't what I want. I ordered a made to measure suit off of Thick as Thieves (which gets a lot of buzz on here...search it on the forum) a few days ago and am looking forward to seeing the results.http://www.thickasthievesla.com/
Don't forget shoes though. Bad shoes can ruin everything when you are building a more professional wardrobe. I got a pair of Cole Haans a for a little over hundred at Macys recently that I think look pretty good. When I have a bit more money, I'm going to go with some Allen Edmonds.
Shoot me a message if you have any more questions.