The US site of New England Biolabs has pre-made and assayed E. coli Topoisomerase I listed as an available product. I don't know if it's available at the Chinese subsidiary (http://www.neb-china.com/) because whenever I try to check it redirects me to the US site, but even if they don't have the enzyme you could see if they can import it from the parent company.

Technically I *think* if you linearize the plasmid with a single cutting restriction enzyme then re-ligate it you should also get relaxed circular DNA, but you might also end up with some residual unligated vector and maybe some plasmid multimer ligation products as well.

Oh I also just noticed this text on NEB's site:

"FAQ: If the DNA resulting from a Topoisomerase I reaction is run in an ethidium bromide gel and there seems to be no evidence of enzyme activity, what is the most probable explanation?

Ethidium bromide intercalates between the bases of the DNA and induces supercoiling. All relaxed plasmids regardless of linking number will become supercoiled in the presence of ethidium bromide and appear to run at the same position equivalent to any supercoiled DNA in the sample. Therefore to check the result of a Topoisomerase I reaction, the products MUST be run in a gel without any ethidium bromide. After the gel is run, it can then be stained with ethidium bromide to visualize the DNA. The relaxed forms of the plasmid will run slower and at the top of the gel. Any supercoiled plasmid will run faster and lower on the gel."

Source: https://www.neb.com/en-us/faqs/2011/05/30/if-the-dna-resulting-from-a-topoisomerase-i-reaction-is-run-in-an-ethidium-bromide-gel-and-there

Plasmid can be relaxed by nicking it with a restriction enzyme, then resealing the nick with DNA ligase.

A type 1 topoismerase should also work, although you will have to titrate the enzyme concentration to get maximal relaxation. Here are 2 methods we used (https://www.slas-discovery.org/article/S2472-5552(22)07985-0/fulltext):

Preparation of relaxed plasmid

Supercoiled triplex-forming plasmid used in some experiments was purified with a Gigaprep kit (Qiagen, Valencia, CA) and relaxed using vaccina virus topoisomerase 1 (Sigma-Aldrich, St. Louis, MO). The relaxation reaction contained 0.25 mg/mL of plasmid and 0.86 U/mL topoisomerase 1 in a solution containing 39 mM Tris-HCl (pH 7.5), 42-mM NaCl, 4.3% glycerol, 0.31-mM EDTA, 2.5 mM MgCl2, 86-µM dithiothreitol, and 0.0086% Triton X-100 detergent. The mixture was incubated at 30°C for 2 h. The relaxed plasmid was purified by ethanol precipitation and resuspended in buffer consisting of 10 mM Tris-HCl (pH 7.5) and 1-mM EDTA.

Relaxation of high-purity, supercoiled triplex-forming plasmid prepared by Nature Technologies (Lincoln, NE) with E. coli TopA was performed as follows: highly purified plasmid at 2 mg/mL was combined with 60-nM E. coli TopA in a buffer consisting of 20-mM Tris-HCl (pH 8), 50-mM potassium acetate, 10-mM magnesium acetate, 2-mM dithiothreitol, and 100-µg/mL bovine serum albumin and incubated at 38°C for 2 h. Because the TopA becomes inactivated during this procedure, repurification of the resulting fully relaxed plasmid was not necessary.

Plasmid prepared by each method behaved similarly. The method of preparation of plasmid used in each experiment is given in the figure legends.

Here are two papers on using human type 2 topoisomerases to relax plasmids.

https://pubmed.ncbi.nlm.nih.gov/24309019/

https://pubmed.ncbi.nlm.nih.gov/23415903/

A relaxed circular plasmid can be prepared by treating supercoiled plasmid DNA with a nicking enzyme (such as nicking endonucleases) or by partial digestion with DNase I under controlled conditions. This introduces single-strand breaks, relieving supercoiling without fully linearizing the plasmid.