For the accommodation I don’t think you are going to find a single source outside of those that have been mentioned. I think you will do better on cost if you contact the apartment owners directly.
How do you do that? Go to AirBnb and find a few places you like and look for some unique wording in the description, then google those words in quotation marks and with some luck you might find the owner’s private listing or other platforms that handle the unit.
Another way is to contact local guide companies. They often have units that they represent or recommend. I can maybe give you some leads in Ukraine, Croatia (also ask TexasTravelMom), Bosnia & Herzegovina, Montenegro, Hungary, Bulgaria and Romania.
So you didn’t ask for this, but EU - Europe, UK and non-EU eastern Europe means it can be done without Schengen problems. I am sure you know this, but EU does not equal Schengen and vice versa. Something like this but pad the out of Schengen a bit to stay out of trouble.
90 days Schengen
Austria, Belgium, Czech Republic, Croatia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland.
90 days UK
England, Scotland, Wales, Northern Ireland
90 days Europe outside of Schengen
Bulgaria (EU), Romania (EU), Cyprus (EU), Ireland (EU), Albania, Armenia, Azerbaijan, Belarus, Bosnia & Herzegovina, Kosovo, North Macedonia, Moldova, Montenegro, Serbia, Ukraine
90 days Schengen
Austria, Belgium, Czech Republic, Croatia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland.