Find a scroll of plane shift
Plane shift is still a viable option for planar traveling at lower levels, as long as you can find spell scrolls of it (or a high level wizard NPC that scribe such scrolls). The scroll can be used by low level clerics, druids, sorcerers, warlocks and wizards, as the spell scroll description says (Dungeon Master's Guide, pg. 200):
If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.
The DC 17 is kind high, but you can combine effects like guidance, enhance ability, and Bardic Inspiration to make your life easier.
Find a Horizon Walker Ranger
At 3rd level, the Horizon Walker ranger archetype gains the following feature (Xanathar's Guide to Everything, pg. 42):
At 3rd level, you gain the ability to magically sense the presence of a planar portal. As an action, you detect the distance and direction to the closest planar portal within 1 mile of you.
Once you use this feature, you can't use it again until you finish a short or long rest.
See the "Planar Travel" section in chapter 2 of the Dungeon Master's Guide for examples of planar portals.
This feature is the best option you have to locate a "natural" planar portal.
If you want to travel to a specific plane, blink and banishment can be combined to travel to the Ethereal Plane, as explained here. If you want to travel to the Astral Plane, you can throw a bag of holding or similar item into the extradimensional space of a portable hole or Heward's handy haversack, but both items are destroyed in the process.