I might be getting $500 for a photo gig shortly and I want to use it to get a macro lens. What's my best shot around that price range? I don't mind supplementing a couple hundred if it's worth it.

Canon EF 100mm 2.8?
Sigma 150mm 2.8?

Sigma
Both lenses are great, but the extra reach on the Sigma helps. You might be able to find a used one for around $500.

Lani has spoken. I was leaning there anyways.

What about accessories? Anything to make my life easier if I get it?

External flash and diffuser, but that'll cost you more, lol.
Tripod.

Got the tripod and the speedlight 430 - i'm not great at using the speedlight though.

http://www.lumiquest.com/

Ooooooo...now i have to research more, lol. Thanks for the link.

Sigma all the way..You get a tripod ring, lens hood and pouch for free..canon all those things are extra and a lens hood is something I would ALWAYS want to have.

Screw those expensive bounce things...Go buy yourself a heavy stock index card...like a 4x5" then just cut it to shape so it fits on the flash with a rubber band. Boom got yourself a nice flash diffuser. I have been doing it that way for years. you can see the shape I mean to cut it here in my bag

Good point. Decision made. Now i just hope i didn't count my chickens before they hatched...

So the diffuser allows for most light to hit the ceiling but some of it to be reflected onto the subject? What is the purpose of doing this when shooting my tank? Or is this just for other subjects in low-light?

well when shooting tank..it will put the light source above the camera enough that will help to stop getting the flash in the picture and hot spotting the image.
plus its a softer light and not harsh so the shadows will be much softer and not hard edge shadows.
shot this image with my little home made diffuser

Personally I try not to use a flash on a tank as much as possible..I kinda hate the way it makes everything look fake unless you can fill with it properly and not over power the tank lights.

Gotcha - I can see how it would be useful for shooting fish. Alright, i'll give it a try.

noob question, but does the sigma lens have anything equivalent to canon's USM ring motor?

The Sigma equivalent of USM is them HSM. This 150 macro does have this.

-Shawn

but there's 2 USM... the ring type and i guess the micro motor? type... could be wrong, if so someone correct me

yes canon has a Ring type and micro..the micro is in the cheaper lenses with USM..like the 50 1.4 and such..the L's and the higher quality EF lenses that have USM get ring.
Sigma has HSM that is the equivalent of the canon USM and I am not sure if they have 2 versions..I think they only use ring. I will say HSM is very good and right there with canon when comparing lenses of the same caliber

anyone know where to find the sigma 150mm used? b&h doesn't have any and i'm not sure what other sites are trustworthy.

check out fredmiranda.com or photography-on-the.net classified section.

Not sure how easy it is to join anymore, but FredMiranda.com has one of the best collections of top-notch professional photographers around. I've bought a 20d, 17-40 F4L, and 70-200 F4L off of their Classified sections with no problems.

-Adam