That depends upon your Budget mate
You can purchase some pre-made scripts for few hundreds
Or you can hire few programmers to do that according to your requirements which will cost you thousands
Topic: how to get a good BTC trading site script? (Read 389 times)
You means trading bot?
You can check my thread: https://bitcointalksearch.org/topic/bitbot-the-bot-trading-for-the-indonesian-bitcoin-community-1452338
You can check my thread: https://bitcointalksearch.org/topic/bitbot-the-bot-trading-for-the-indonesian-bitcoin-community-1452338
Do you mean automated trading cryptocurrency, if you still new in trading better to not use automated trading service. you need to learn manual trading to get more experience.
IF you mean a exchange portal you can develop it from scratch if you know PHP. There are other sources where you can buy an exchange. but just think of the problem when they have a back door...
how to get a good BTC trading site script?
Are you referring towards exchanger script to open trading platform or just a bot script for trading? 
how to get a good BTC trading site script?
Do you mean like a trading bot which automatically does trading for you with a few clicks? If so, I recommend that you use CAT by sampey: https://bitcointalksearch.org/topic/m.5590877. It does cost quite a bit but the software is very very good. If you buy it, please tell Sampey that I referred you. 
how to get a good BTC trading site script?
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