For your f/5 newtonian, the shortest you could use (high power) will be 4-5mm, the longest (low power) 30mm.
For budget ones try
www.andrewscom.com.au, under Optical/Telescopes/Guan Sheng there's a reasonable range for $39 apiece.
Plossls are usually surprisingly good with Newtonians, the other thing to watch for is a set of Edmund RKE's (8...28mm) if you find a set secondhand, these were designed to match small newtonians around f/5. You can still buy them new from their website.