Good question. So far Fidelity is the best broker for individual muni bond,
no fee for IPO
www.fidelity.com/newissuemunis
If secondary, $1 for $1000 bond for your bond number more than 8( face value
is above $8000), otherwise $8. For muni, minimum amount is 5 or 5000$ face
value, can be less if you buy a zero coupon bond.
However, there's no broker you can easily sell your muni bond without some
spreading. Because most muni bond holder will buy and hold until maturity.
If you want a small spreading, try General Obligation bond of large state
like CA, or Puerto Rico bond because no state tax for all investors, or
large scale transportation bonds like over $1 billion size. Buying a muni is
easy for retail investor because many of the ask price accept 5 or 10 bond
(5000$ or 10000$ face value). But selling is hard because the broker has to
buy the bond by itself first. If you really want better liquidity, try
corporate bonds first because they can be sold online like stocks.