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You can get SMA RF relays from ebay but they are not that cheap anymore... ~£25 for 12v operating ones, sometimes less for 28v versions.
An alternate option is to use a diplexer with one antenna for both Tx and Rx as long as they are on different frequencies - repeater use.
I have some diplexers available (they are quite big!) the only problem there is that you are fixed to certain frequencies.