Best option is to get a downgrade service or carefully downgrade yah ps3 if you know what yah doing using E3 Flasher. The reason why you shouldn't get a used 3.55 PS3 is because you can get one that is banned and is not that good as your original unique ID is special and is not being spoofed using a PSID patch which the Console ID may be released to the hundreds who have CFW
look on ebay for one, the cheapest i've seen is around $280. but I would suggest doing an e3 flasher service for about $60 which downgrades your ps3 to 3.55