I've had a lot of success with FB marketplace with my rentals.

If you have to save some $ somewhere: Get some exact measurements and go find a used , surplus, scratch and dent, fb marketplace, etc -- tub that fits (double tripple checking specs), for your contractor to install.

Strongly consider using a Certified 203k Contractor. 

Sites like Craigslist, Facebook Marketplace, and Roommates.com can give you a good idea.

Looking for a "Board Certified Attorney" related to real estate would be your best bet.

Too few details to answer (Of course this varies county by county, all questions could be answered by reading the (zoning)/building code and/or clarified with a visit to the county planning office).but some clarifications because: lots of things are "tiny homes" but what you describe small houses "on permanent foundations" needs to be clarified , tiny homes could be:-stick built, see below. check your county code/planning dept-stick built cabins/recreational, a specific zoning and potential occupancy/etc restrictions but worth looking into where the zoning allows for it -hud manufactured homes (smaller singlewide perhaps 400sq ft 600sq ft minimum)-non-hud park models (on axles), ansi standards- other "RV's" built on a chassis:  organizations, state agencies, etc - will certify factory built (or sometimes inspected professional built) tiny homes on chassis - to whatever standards ansi, riva, dot/ l&i?

Do they have relevant credentials, like Certified Investor Agent Specialist (CIAS)?

Here are a few places to post in search of rent by the room residents:1.Craigslist.2.Facebook Marketplace.3.Zillow (They recently rolled out the room rental listing.)4.

Is your money with a certified 1031 exchange holder now? 

You likely won't find your answer on the internet, FB marketplace is a good start, but I found all my help up there through referrals and picking up the phone.