On the subject of camping, both reserved and free (informal), the Isle of Man Government has this to say:
There are a number of registered campsites around the Isle of Man.
Informal seasonal camping is also permitted at 2 venues managed by the Department of Environment, Food and Agriculture:
The Sulby Claddagh is not a registered campsite, but informal camping is allowed Easter Weekend and then from 1 May to 30 September. Basic facilities are provided onsite including toilets, but there are no showers.
You will need a camping permit to camp overnight at the Claddagh.
Permits are £15 and you should apply for one well in advance of your visit as permits will not normally be issued on site. Permits may only be obtained by a responsible adult aged 18 years or over.
Permits are valid for a season – Easter Weekend, and then from 1 May to 30 September. Camping on the Claddagh is not permitted outside this period.
A permit allows the holder to pitch a camping unit (motorhome, tent, campervan, caravan, trailer tent, etc) staying for up to a maximum of 14 nights at a time. Your permit must be clearly displayed at all times when camping overnight. Units must be in continuous occupation.
A permit also covers 'pup' tents associated with a family from the same household.
A permit does not reserve a pitch at Sulby Claddagh. Pitches are available on a first come first served basis so space can never be guaranteed. Community events held on the Claddaghs may further restrict space for camping on certain occasions.
Informal camping is also permitted in the Recreational Area at the Ayres National Nature Reserve. There are no toilet or shower facilities here and camping in tents is not allowed.
Camping is restricted to a maximum of 15 motorhomes for a maximum of 3 consecutive nights.
Visitors are asked to respect this special place and leave the site in the condition that it was found.
Campers must comply with the Ayres Byelaws, a copy of which may be downloaded from this
Edit from original question asker:
I was able to finally find a place in advance online at the Foxdale AFC campsite. Even in the middle of the TT, most, if not all campsites I saw appeared to have plenty of room for more tents and I'm sure they'd be helpful in taking in additional people.
As for getting around on foot and public transportation, location is everything! Study the bus routes well in advance, as some campsites are near a bus routes, but runs only a couple times a day. Being close to the routes between Douglas and Peel or Douglas and Ramsey ensures buses both run frequently and later into the night.
Lastly, it should be noted to anyone not familiar with Isle of Man. I didn't get bit by any mosquitos, however midges, a bug I wasn't familiar with was a huge nuisance. They seem unaffected by bug spray, swarm around you like mosquitos around dusk, and their bites actually hurt a little.