You can still easily buy it here, but the over-the-counter pills are always mixed with other ingredients to make it more difficult to convert them into amphetamines.

E.g. Contac 600 Plus can be found in basically all drug stores and it has 120mg of Pseudoephedrine, 100mg Caffeine, 8mg Chlorpheniramine, and 0.4mg of Belladonna Extract. It sounds like it'll actually be illegal to import into Japan, since 120/(120 + 100 + 8 + 0.4) is over 10%, but I've previously just walked into a drug store and bought a packet.

Probably the same thing you do in the states if you have high blood pressure: make do with lesser medications, pain killers, lots of liquids, and push through it.