Hi,
Beside Rauol's suggestion, you may use the well-known astabil multivibrator built from descrete bipolar transistors with 50-60V minimum collector-emitter breakdown voltage.
see such a page at **broken link removed** where you can consider your individual low collector currents and your frequency in the formulas given. It is sure that several Megaohm resistors will be involved. Of course you have to choose a suitable transistor type (the Vcemax of BC107 is 45V, maybe enough).
Another practical solution would be to use the low power circuits on their usually low voltage supply given in the earlier posts and use a low current descrete bipolar buffer amplifier run at your 20-30V supply at your uA collector current.
rgds, unkarc