Check the wiki page for agony for a complete table of damage taken from agony in relation to fractal level and agony resistance. It starts off with minimum damage at 10 ar at level 10 and increases by 15 ar every 10 levels. That way you can go up to level 39 with trinkets only if you get infused rings/backpiece and/or craft agony infusions higher than 5 for the extra infusion slot of your rings and backpiece.
The easiest way to get agony resistance is probably buying the +5ar infusions available in the fractal lobby for 75 relics. You should be able to afford one every 2-3 fractals. Rings can be found in fractals after level 10 (if you don’t care about optimum stat combinations) or traded with pristine relics (you get one in each daily fractal chest, allowing you to trade the ring of your choice after doing 10 fractals). Trinkets and amulets can be aquired at the laurel vendor, guild commendations vendor and/or wvw laurel vendor.
To infuse your rings (adds an extra slot for an additional craftable infusion) you need mist essences that you can only find in fractals, so at what point you’ll be able to infuse them (or even lucky enough to drop an already infused ring in a fractal past level 20) really is a question of random luck, but most people have one or two infused rings available by the time they reach fractal level 30 and need the extra ar from that slot.
If you want to go beyond fractal level 39, ascended armor and/or weapons really is the way to go, since crafted infusions with high enough ar to make up for the missing armor slots are insanely expensive.
