Any differences in price get arbitraged to death.  Their prices are going to be pretty much the same for practical purposes.  It's worth looking on coinmarketcap for trustworthy exchanges (none of them are that trustworthy, so be careful), but Bitstamp, GDAX and Kraken are a few that you could try.
If you want to pay with cash, you should probably try trading with someone in person (or with cash in the mail if you need to) on LocalBitcoins.
You're missing out bank transfers, which can be quite important.  Between cash and credit cards though, cash is ideal since it's non-reversible.  The risk of credit card fraud and necessary KYC regulations lead to a lot of complex verification which can be quite irritating.  

Credit cards are ideally for when you need a convenient way to pay online, but they can also result in high fees and annoying verification.