You will have very limited liquidity if you don't invest in at least a multiple of 10k. It is true that your liquidity will be somewhat impaired with $20k or less even if you put it all in one bond, but it's a matter of what you're comfortable with. I would not consider owning less than 10k of a bond, or investing between 10k-50k in anything but a multiple of $10k.
I agree that muni mutual funds are reasonable alternatives for you. You may also look for a site that promotes direct bond transactions between individual buyers and sellers -- there are some that operate almost like EBay. This kind of site allows you to buy small chunks of bonds even cheaper than an institution could buy $10mm of the same bond, but in my experience they are not a great place to try to sell bonds. Worth looking into if you can buy a 10k chunk there, take physical delivery and then sell through some other conduit, but I dont know how feasible that approach is.
If you can find a muni fund with expenses under 20-25 bps, that would be a good choice.