I heard that IP spoofing is now mostly blocked by ISPs, especially on the consumer-grade links. Is there any research on the number of networks that still allow users to spoof an IP packet with an address outside originating ISP?
Yes there is some statistical data about that: http://spoofer.cmand.org/summary.php
// Quoting the referenced Pages FAQ:
Actually, our measurements clearly show that spoofing is still prevalent among approximately 25% of the autonomous systems and netblocks we survey. More importantly, a single entry point for spoofed traffic provides attackers a means to send spoofed traffic to the entire Internet. ISPs can employ filtering [RFC2827] to ensure their outbound traffic is not spoofed. But there is currently no way to ensure that inbound traffic is legitimate as long as there exist entry points for spoofed traffic. uRPF [RFC3704] does not work, and is not used, in the core of the network where routing asymmetry renders it useless.
Here are some testing tools that offer up the capability to test your ISP for yourself -- https://web.archive.org/web/20110721053646/http://rmeijer.home.xs4all.nl/spoofaudit.html -- http://spoofer.csail.mit.edu/ -- each will provide some mechanism to detect if IP spoofing is possible, although you may need at least two nodes inside that specific ISP in order to test the two-way characteristics of said IP spoofing.