So I currently have a Line 6 Spider III 15 and it's pretty bad, I play mostly post-punk, indie rock and 60's rock (beatles) and would like to get a Vox AC30 but I'm not going to be able to afford one so I was wondering is anyone had suggestions for a cheaper amp that can get good sounds for those genres, something $300 or under.

I'd look at the AC4TV and the Vox Night Train. Both are pretty decent amps in my book. The Night Train will be more versatile as the AC4 is kinda one dimensional. You also might be able to find a used AC15 in your budget'ish
I would definitely check out the AC4TV based on your budget and genres, along with the others mentioned here for sure. I can only speak to the AC4TV though as it's the only one of those I own. As 311 said, it "is kind of one dimensional". I agree. I've found it takes pedals ridiculously well though (imo of course), so it's at least worth a look
Main Gear:
Gibson Les Paul Standard
Fender Telecaster and Stratocaster
Dr. Z Maz 18 Jr. NR, Vintage Fender Twin Reverb (early SF)
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Session Guitarist out of Pittsburgh