Ropuchy,
Have you considered using a reversed lens or microscope objective on extension tubes ? Depending on how much magnification you require it can be an inexpensive solution.
I'm experimenting with that myself. I recently picked up and Amscope 4X microscope object and mounted that on my bellows to shoot snowflakes. I picked up an RMS/M42 adapter which made it very easy to mount and I was very happy with the results.
Its designed to be used with a 160mm tube and when I mount it on that I get a horizontal FOV of about 5.5mm i.e. 4X.
I then tried reversing some of my Pentax lenses on the tubes using a reversing ring.
55mm gave me FOV of 9mm (2.6X)
35mm gave me FOV of 5mm (4.7X)
28mm gave me FOV of 4mm (5.9X)
15mm gave me FOV of 2mm (12X)
They all had pretty decent working distances, surely compared to regular microscope objectives.
I upload a test shot to Flickr
Top image was with a Pentax-A 100mm F2.8 Macro (1X).
For comparison the Lego 'button' is approx. 5mm across.
Hope that helps.