What are you planing on doing with it exactly that you need another IP? I assume Firewall and perhaps IPSec termination in which case NAT will bork your IPSec with a ADSL Router in front unless both ends do NAT-T. The PIX does not support PPPoE or PPPoA (oddly the 501 supports PPPoE though, go figure) so your crap out of luck getting an ADSL router and putting it in bridging mode..
Try a ADSL router setting up a DMZ to the PIX and if the IPSec is the problem check NAT-T support for both devices.
If you need extra IP addressing space you need to justify it unfortunately, if ISP's go wasting IP's with /30 subnets or allocating large subnets when not needed it makes it hard to justify with APNIC or the regional authority if they decided to look at your IP address space usage.
If you would like message me and I could provide you pricing on this either as a routed /30 or /32 to loopback or other internal setup.
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