Matt has recommended Multicharts (which is able to use Tradestation code) as an alternative to Tradestation.  You would then use a broker such as IB.  I have no experience with it, just thought I'd pass it on.  Something else I don't have any experience with is Cool-trade (also linked to a broke).

I believe many members use Tradestation as broker and platform (although TS has been reported as having difficulty finding shares to short most of the time). You can get around this by using Trade bullet to send triggers to your broker. I have only heard of this, I have no personal experience. You may also use Ninja Trader as a platform with your broker.