If you sheared off the bleeder, maybe you can extract the remaining bleeder threads that are still in the caliper using an extraction bit and replace with a new bleeder screw? You have already talked with a mechanic so they probably would have suggested it if that was an option, but it would be what I would try first if I sheared off a brake bleeder.
One affordable place that sells brake stuff is centric, and they have a front caliper for a 98 Boxster.
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Comes loaded with pads - I am not sure which ones but if you are using your own pads you could buy the variant with the cheapest preloaded pads and throw the pads away. Around $110 with a $100 core charge it looks like, and they are probably rebuilt units using their caliper rebuild kits. Note I have never gotten one but from looking at their info it looks like a rebuilt brembo caliper.
I would not foresee an issue with one old caliper and one new one, but I would probably defer to someone that had more experience.