Where are the roasted pepitas?

Roasted Pepitas comes from carving a Pumpkin (which gives you five) as well as smashing a carved pumpkin (which gives you an additional five).

While the Pumpkins, Pumpkin Seeds, Pepitas and the recipies stay after Zilloween, the carve verb, the Pumpkin Seeds in the Gardening Goods Vendor and the ability to plant them are only available during Zilloween. Any carved Pumpkins, unlit and lit, decompose into Pepitas and Pumpkin Pies after the event.

Zilloween is a holiday that happens every 37th of Remember every Glitch Year. Since time in Glitch passes six times as fast, and there are 308 days in every Glitch year, the event occurs every 51.3 Earth days. The period of time in which the holiday items are 'active' lasts several real world days.

As to answer the actual question, the best way to get Pepitas off season is simply to ask. Glitch is, ostensibly, a social game, and the players are generally very friendly. Asking on Global, group or even local chat will usually get you someone who is willing to trade or give you the item you need. Supply of items on auction can be finicky, and you often get overcharged for rarer items like Pepitas.

From the Glitch wiki:

Roasted Pepitas are a seasonal Food: roasted Pumpkin seeds. They are acquired by carving a Pumpkin, using Knife & Board and a Pick. Carving one Pumpkin gives 5 Roasted Pepitas (and a Carved Pumpkin).

Pumpkins are a tasty seasonal crop. They can only be grown or carved during Zilloween. You can grow these in a crop or a herb garden. Pumpkin Seed is available at Gardening vendors.