A couple of extension tubes on the back of your 135mm Tamron will probably be all you need for flowers, and probably the cheapest option ... close-up "dioptres" would be another alternative, though the 58mm filter size will push the price up a bit.
After that, macro doesn't usually come cheap, the Russian N-61L/Z in M42-mount (often called an Industar) can be well priced, but you'll need an M42-PK adaptor. As you'll be using it for macro, infinity focus shouldn't be an issue, so you can go for one of the "flanged" adaptors which are available for "not-a-lot". Leave it fitted to the lens and you've got a fully-manual 50mm macro lens in PK mount ... don't forget to get a lens-cap
I always recommend the Tamron 90mm, but you'll need to get lucky to find one in budget ... at least you've already got the Adaptall adaptor
Good luck