Bitcoin is not differ in price at different places but you may be charged a little more on some exchanges.
The world economy is completely digitalized as of now, still you want to pay by cash?
I would recommend buying by card instead of looking cash deposit centres.
If you change your mind to card payment option, then coinbase is the better website to buy bitcoins using card.

but i think we can only choose the top 100 if we are new person because we need to understand the basic learning about trading and its only learn by doing in trading and keep practice. i think the top 50 is enough for us to still continue learning trading because there is many people will interesting too with bitcoin and no one can share their method to reach the profit. if you can do with 50 coins then i am sure that you can watching the price is up and down so you can make your own decision to buy at what price.

вы бы вопрос сформулировали точнее. вам нужно знать стоимость битка в сша? так он будет немного отличаться от курса на бирже.

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.

https://coinmarketcap.com/exchanges/volume/24-hour/
This site won't exactly tell you all the exchanges but most them listed here are old and you can trust it little bit. My recommendation would be use bittrex , poloniex and maybe hitbtc if you want.
I wouldn't recommend you sending cash directly because sometimes they screw up and you will lose your cash and not get your btc also.

Both have their disadvantages as i said cash transactions can't be reversed ans most of the trusted exchanges above don't accept cash directly. For credit card it is safer because of chargeback feature but the fees is as high as 20%. Either what you can do is register on https://tether.to verify all your documents and address and send funds using your bank account. After funds arrived you can use it on the above exchanges without any more fees.
If you don't want to provide any documents just use local bitcoins you might find any local trader and it is more easier and safer to trade.