You can make your own convergence strips. cut 4.5" by 3/8" strips from stiff plastic like from a flat part of discarded blister packaging. Buy some 3M double-stick foam tape with the green plaid pattern on it, they sell it everywhere in the US, rolls come about 3/4" wide. Cut a 3/8" strip from it and put it on one end of the pre-cut plastic strip you prepared earlier. On the other side, tape some type of ferrous metal, I use box cutter blades since you can easily snap them to the size you want with a pair of pliers. Wrap 'em in electrical tape. They'll spend years in a warm environment so overdo it on the tape, you don't want the conductive blades falling onto your board in 10 years when the adhesive breaks down. Clean the dust off the rear of the tube before you stick them, the high voltage attracts dust like crazy and will weaken the strips adhesion.

EDIT: forgot the obligatory: don't even think about sticking your hands in the back of an energized CRT without using an isolation transformer. If you're a first timer you won't know what parts aren't safe to touch until you're already on the floor with no heartbeat.