Alec,
You can usually buy a pair of 'close' glasses in your prescription for a reasonable price. Bifocals aren't the ideal for 'tying glasses'.
However, before you do that, check to see if ALL you need is some extra magnification. I've always needed lenses for 'distance' things like driving, etc., but until recently my close up vision was fine. Hit the big 40, and started to notice that I was having trouble tying knots on the water, then seeing stuff well at the bench.
Ran to the eye doctor, convinced I had some strange and incurable malady and was going blind.
He looked at me, AFTER the exam, and said "nothing wrong with your eyes, you're just getting old. Go buy a cheap pair of magnifiers for your fly tying and see if that does the trick".
Worked for me. I did go through several pair at around $3-$7 each until I found the magnification and frame fit I liked.
Good Luck!
Buddy
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Tie flies beyond the resistance of mortal trout!
It Just Doesn't Matter....