Check out Garmin's GPS-18. You can "Buy It Now" on eBay for ~ US$80.00. Look for "GPS 18 OEM." I've had mine connected since the BCT15 first hit our shores and absolutely love it!
Be aware: the GPS-18 comes in multple varieties. The "PC" version has a 9-pin serial connector and cigarette lighter power adapter. If the serial pinouts are a match for the BCT15 -- I'm not sure -- then it's plug-and-play. The "LVC" version has open-ended wires, for fabricating your own solution. There's also a "USB" version, but this'll do you no good.
I purchased the LVC version for hard-wiring into my car, but ultimately would've saved some money had I gone with the PC version and chopped 'er up. With the LVC version, I had to purchase a step-down transformer to go from the vehicle's 12V to 4.5V; $30 at RadioShack. This is built into the power adapter of the PC variant.
I hope this is helpful.
Dave