The only way to go tubeless, which is an excellent idea by the way as you can all but forget about punctures, is to fit tubeless wheels. You cant usually fit tubeless tyres to powerchair wheels. At which point your tyre sizes need to be whatever the wheel dictates, and about the same overall diameter.
Your current sizes dont actually tell us what size rims you have.
The 145/70 - 6 tyres (all terrain) that I use are actually 6 inches wide... Thats a foot of rubber in total on a 2 foot wide vehicle. So fitting those means either a really wide and unusable wheelchair or serious modifications to move motors inboard etc. They are on ebay for 49 pounds including a wheel for a pair at times. Fitting 3.50 x 8 tyres can be done easily. Even on existing rims although they will be pinched slightly and will not be tubeless.
Theres 3.00 x 4 (3 wide and 9.5 diameter) tyres on ebay.
http://cgi.ebay.co.uk/Tyre-3-00-4-petro ... 4cf22dfa00There are few right now but usually loads.
I used to buy 8 at a time with tubes for 40 uk pounds... And tubeless tyres in 3.50 x 8 on
http://www.monkeybike.co.uk including tubeless.
http://www.monkeybike.co.uk/acatalog/3. ... _Tyre.html not cheap though...