Hit up dfwmazdas.com for more local info.
For $500 or under you will have to live outside of the 635 circle. You can prob find somehting in that range in north Carrolton, north FB, or maybe Lewsiville.
If you are willing to go up about $200 a month you may be able to find somehting off the North Dallas Tollway betwen Frankfurt and 191 - tons of apartments in that area with a lot of working class young professionals. Also Addison has a ton of apartments and is really close to Carrolton & FB.
You can also check out lower greenville ave. There are a lot of duplexes for rent there, but the area is a high party part of town and 5 minutes from downtown so you may not find anything for that price range.
I would check out:
powerproperties.com - they redo old mansions into really nice and stylish aprtments
craigslist.com also usually has some great listings.
Really the only part of town to stay away from is south of Dallas - Oak Cliff.
Anything north of dallas is safe and will have a easy commute for you if you are goign to the carrolton/FB area.