I would not encourage anyone to acquire any opioid medication not prescribed, for a number of reasons. That said, Tapentadol - a cousin of tramadol but much stronger - can be easily bought from Indian and other Asian pharmacies on the clearnet and at affordable prices. One might have a package here and there get seized but all that results in is a snide letter from customs.

According to wikipedia the reason tramadol has a ceiling point is because it depends on the number of CYP2D6 enzymes. This can theoretically easily be bypassed by co-administrating a CYP2D6 inhibitor however doing so require research and care about dose since a CYP inhibitor can in some contexts upregulate a drug dose potency by 1000X