And in response to the original poster, I would like to make two quick points:
1.) Renting would work. Be sure not to use the network upgrade option since Sony only supplies the newest version (that's 2.0 for Japanese PSPs and 1.52 for US PSPs). Be advised that the 2.0 firmware is 16.3 Megabytes in size (you want to avoid that). I don't know the file size of the 1.51/1.52 updates off the top of my head but I'm sure you can find them through Google.
2.) Please make sure that you really need 1.50. There are plenty of ways to get software to run on 1.00 PSPs. If you have a 1.00, there's usually no reason to upgrade to 1.50 unless you own a game that requires an update.
The safest way to check if a firmware update is valid (ie. not a hoax, like the supposed downgrader) is to verify the MD5 hash. If you don't have a reliable source to compare your hash to, you can check the filesize.