you can use the cme forex futures. it's not very liquid but basically it's like trading forex from futures contracts (with DOM).
but the forex spot are OTC type products. you are obligated to pay the spread even if you enter with limit orders.
for example if you enter a buy limit at the bid your order will only be executed if the ask price hits your entry point.
In short, limit orders under forex are just market orders.
TWS IB have a DOM forex