Hi there, to answer your question, an inexpensive 2x Barlow will serve you well. You can find them new for £20+. With your 10mm you will see what 150x mag is like. You can also back off a bit by removing the Barlow lens and screwing it on your 10mm eyepiece to give you around 112x mag. Likewise you can do the same with your 25mm to see what results you get on different targets. Take it slow and use what you have. When you work out what you like, and what your conditions allow, you can make a better informed decision about eyepieces to get further down the line.