Have you tried searching with the calendar/flexible date option on the UA, CO, or US sites?

Some posters above suggested using the ANA or AC sites, but those are more advanced tools. It sounds like you just want to redeem for domestic flights on UA or US, which either airline's web site should work just fine.

Don't forgot that UA miles (with some exception that is not likely applicable to you) can be used for US and CO flights under their own saver/lowest mileage lower. So, you can use US and CO to check on awards, and then use united.com or call UA to book.

Also, if you want more help, be specific (dates/destinations/cabins) about what you are looking for, so other forum regulars can give you more concrete suggestions.