Double-check, but here's a list of options you have :-)

1

Find another $10-$20 telescope on Craigslist that has a diagonal and eyepiece. Sounds dumb, but it's sometimes your cheapest option.

What area are you from?

 

Older eyepieces, often included in such sets, are Kellner and Huygens. Not the best quality, contrast, small apparent field of view, short eye-relief.

Old telescopes had a 0.965" focuser. You need an adapter to use more modern eyepieces.

 

2

Cheapest

Warning: Poor performance. You might loose interest just because of that.

0.965" eyepiece Set With Diagonal - You can save a few bucks with sets that do not have the diagonal, but if you look up without one, you'll be breaking your neck! :-)

=>

700mm focal-length divided by eyepiece

• 20mm = 35x magnification (Overview)

• 12.5mm 56x

• 6mm = 117x (scraping at the maxmum possible with this telescope. Dim. Short eye relief.)

Huygens eyepieces are one of the simplest designs. It's basically just two plano-convex lenses. Short eye-relief, distortion, chromatic aberration, small apparent field of view.

More magnification does not equal better views. More means dimmer. In small telescopes like this, over 100x is pushing things. A larger aperture telescope will show more at the same magnification, as the image isn't as dim and dull.

 

3

Higher budget

Better alternative;

• 0.965"-1.25" diagonal (if you have a 0.965" focuser) eBay/US, eBay/China,

  • 25-32mm Plössl for the larger/largest overview, $19@Amazon, $11@eBay/China, $12@Aliexpress
  • 6mm "gold-line" for best quality (scraping at the maximum of this telescope, but useful for any future telescope as it's a great budget eyepiece) $19Aliexpress, eBay/China, ($40@Amazon)
  • one or two in-between. E.g. 15mm Plössl or another "gold-line"

These eyepieces will still be useful in most future telescopes :-)

 

4

Second cheapest, budget - Not ideal, but better than the ultra low budget solution

A barlow doubles (or trippels, or...) the magnification of any eyepiece.

Sounds great, but it will reduces the contrast a bit.

It sounds like it saves a lot of money, but 1) the contrast will suffer, and 2) it can bea bit redundant, not saving any money. (e.g. You have a 25mm and 12mm eyepiece, a 2x barlow will only be useful for the 12mm as the 25mm will be similar to the 12mm.).

To save money, a single overview-eyepiece plus barlow for planets works... But you'll be missing an eyepiece for in-between. See the previous solution for a better coverage.

• A) 32mm+5xbarlow

• B) 20mm+3xBarlow,

... but you'll be missing a "medium" eyepiece.

(You can save some money by ordering from Aliexpress or eBay/China. Avoid even cheaper Barlows in general, plastic lenses! Especially the many kits with barlow are basically garbage and not even worth using for casual observing).

 

Finder Scope

You're missing a finder-scope. A red dot finder under $10 works best 1 2 3, as optical finder-scopes (5x24 and stuff like that) under $10 are horrible. For $20 you can get a right-angle 6x30 that's at least usable.

Again, for $20 alone you might find a used telescope to salvage parts from.

(Note: Many cheap red dot finders for rifles usually have a tinted sheet of plastic, that's dimming down things for astronomy use = not good)

 

TL;DR: What's the budget? :-)

To get a somewhat decent diagonal, eyepiece and finder, you can easily spend $30-$40. It often makes more sense to find something used and swap out parts. For $70-$80 you can sometimes get a refurbished Orion Dazzle, with a 4.5" aperture, so spending too much on accessories isn't always the best strategy. In a way, the cheaper eyepiece set is the smartest financial strategy, unless you plan on getting a better telescope later where you would use the eyepieces anyway.

//EDIT: Deleted some redundant stuff, corrected numbers.