You can absolutely use liquid fertilizer. The tree does not really care. Biggest benefit of organic solid fertilizer is the slow decomposition, which results in the tree getting fertilized continuously. The liquid just is there in one wave. I a personally not convinced it flushes out all that quickly, but thats just me. But, with liquid, best is to use a lower dose than on the package, and use it more frequently than on the package.
Solids do not have to be bonsai fertilizer. You can get any fertilizer from the garden centre. Avoid pure chemical pellets and try to find something organic based on bloodmeal or something, as salt pellets will cause problems in pots (damaging the roots due to high salt levels).